How accurate are vehicle odometers?

I think your speedo is deliberately set to indicate a small percentage above your actual speed just to keep you on the safe side of the speed limit.

Al

Lessee. If your actual speed is 30 and your reported speed is 32, then your warranty will run out 6% sooner than it should. Do you drive a Honda? There's been a recall addressing this very thing.

Odometer is the term for the accumulated reading, BTW.

HerbVic,

Thanks. I am driving a Ford Van and a Ford Ranger

It would be nice to think that the vehicle manufacturers were thinking of your safety in regards to speed, but I think not.

I have a 2006 VW Jetta diesel (great @ an average 42 MPG!) that has a speedometer that reads out the same, a couple of MPH over what I'm being clocked. This will cause the odometer to record my distance traveled slightly greater than what is actually traveled.

Now, imagine if the manufacturer inches these readings up on thousands and thousands of their products. They will collectively be clipping off a significant number of miles from their 50K miles of warranty and probably save millions of $'s.

It's a theory I've come up with since the dealer said my speedo was within spec.

Not to mention the fact that it makes you believe you're getting better gas mileage than you actually are.

Hiya,

Having to be aware of cameras in the UK and driving a lot. I purchased over a period two different GPS systems and have found that my speedo in 3 different cars is consistently showing 8% or 9% higher than I am actually doing according to GPS. I.E if the speed limit is 70mph and my speedo says 70 i'm actually doing 63 or thereabouts.

I have queried it and found that speedos are always over rather than under. Can you imagine the fuss if someone got done for speeding when the speedo was showing that they were within the limit!!!!!

I have done the same on all my cars and motorcycles. (variety of manufacturers)

All seem to be at least 3 MPH off.

The exception being my SUV which is running oversized tires, which has corrected it to be dead on and my 2 motorcycles which is are minimum of 5 MPH off. (even more at high speeds and I won't say how high)

My understand is that all vehicle manufacturers do this as a way of eliminating lawsuits. ie. they would be liable for any speeding violations you could attribute to the desing of the car. If you change anything at all then all bets are off.

A car's speedo or as you call it an odometer (which actually is the mileage readout) is very inaccurate.

20 years ago we designed a test rig to test car dashboards for Ford and we had to test the speedo reading for accuracy, the test was at 30 50 and 70 mph...

The tolerance was, if I remember correctly, only -5% but +10% of reading, this was to give a higher speed reading as the average due to speed regulations, so you could be sat on the motorway in the UK doing 77 mph and still only be doing the legal 70 mph... I belive the gearing from the wheel is designed for the wheel to be normally inflated but at high speed temperature so the error is above +10% when you have cold tyres and underinflation...

As the car manual says the speedo is only an indication of your speed not a measured quantity - there are too many variables to allow it to be called a measured value!!!

John.

Let's clear up a couple of things first. The odometer is the group of numbers inside the speedometer that shows how far your car has traveled.

The speedometer is what shows you the speed your car is travelling. This picture shows both the speedometer and the odometer.

The speed indication as well as the odometer reading is affected by your tire diameter. Smaller diameter = faster speed than actual + more distance than actual.

Your tire diameter also affects your mileage. If you keep your tires inflated less than recommended, you lose in more ways than one.

As others have mentioned, the speedometer is not really an accurate indicator of speed. Maybe if all the manufacturers used GPS derived data, then the readings would be more accurate.

Who needs such accuracy anyway? Speedos and Odos have been that way for longer than most of us have been alive and we've had no problem with them being that way.

Your reaction is similar to one of my former company's clients. They commissioned us to build them a bag filling station for filling sacks with sugar. They wanted the thing to stop filling at exactly 60kg. We set it up so that there were two decimal places on the display. The first fill stopped at 59.98kg. They stopped and looked at me as if saying, "well, what are you going to do about this?"

"My God!" I said, "you mean you want zero error all the time?" 'Took a while to convince them to come up with tolerance limits for their filling station.

Contrary to popular belief, tire inflation has little effect on tire circumference, and therefore little effect on the accuracy of a speedometer. Tire selection and tread wear do. Who knows what tires are on the test vehicles when they set up the speedometers and odometers. If they are the same as when compiling mileage data, it would be to their advantage to use tall, skinny, low rolling friction tires for best results. To then put a more normal, better handling (smaller circumference) tire on the production models would result in the reported discrepancies. Just a theory.

Contrary to popular belief, tire inflation has little effect on tire circumference

Your statement is right, under-inflation won't change the circumference but it will change the effective diameter. That being the case, then the other statement, that under-inflation will have little effect on the accuracy of the speedometer, is flawed. Your tires will have more rpms for every meter travelled when it's under-inflated and that translates to lower accuracy of the instrument.

Again, I have to disagree. The treaded part of a tire is in effect a belt (it in fact incorporates steel and/or kevlar and/or polyester strands, or belts), which does not change significantly in length (the circumference I referred to earlier) regardless of tire pressure. The wheel completes one rotation when the "belt", or tread, completes one full rotation. Ironically, there is some heat expansion, and as an under-inflated tire runs hotter (more friction & rolling resistance), the opposite of the commonly believed effect may be true.

These two tires (okay, drawing of tires) are of the same circumference. Due to deflation, the one on the right has a smaller effective diameter than the one on the left. When they spin at the same speed (rpm), the one on the right travels a shorter distance because the chord (part of the circle that is flattened) is shorter in length than the arc of the tire on the left.

I remember this because we had a spirited discussion about this in highschool.

Funny. I just (yesterday) calibrated my speedo on my three wheeler, and used the effective (loaded) radius x 2pi to get the circumference. But on reflecting, I think Munkey is closer to being correct.

The steel (occasionally Kevlar) belt and especially the overlying "layer" used on high speed tires are effectively non-extensible. The steel belts are laid at a bias, but there are always two (or multiples of two) with crossing plies, so for the circumference to change, these belts would have to shear between them (which in fact they do, but the effect is very very slight). However, the very existence of "layer" which is wound onto the tire circumferentially after the steel plies are applied in the tire building machine, argues that the steel plies are not inextensible. In fact "layer" is there on high speed tires to prevent tire growth due to centrifugal force (let's not start that discussion).

But having said that, at ordinary speeds, the tread can be considered to be a virtually constant circumference belt, with perhaps a .5% change in length with variation in pressure from 20 psi to 30 psi. Your difference in radius of 1" is pretty extreme (much greater that the difference caused by changing pressure between 20 psi and 30 psi) but it illustrates your point well.

We know that the flattened surface of the tread really is shorter than an equal (angular) segment from the round part of the tire. We also know that the steel belt area is not much shorter, and that the tread, when viewed from the side, bulges out in a fore and aft direction from the contact patch. If that were not the case, your tire on the right would have a significantly shorter circumference than the one on the left. So, the tread rubber has to squirm quite a lot.

Suppose we use a mark on the tire, (like the M in Michelin) to note how far the tire has rolled in one revolution. (Or imagine inking a line on across the tread, which would print onto the road every time around.) I think you, I and Munkey all agree that, in one revolution of the tread, that "M" will be in the same position relative to the road. Over or under inflate the tire, and that still will be true. We also all agree that the tire does not slip relative to the wheel, so that one revolution of the wheel means one revolution of the bead, sidewall and tread. The tread has a fixed circumference, so one revolution of the wheel also means a fixed distance over the ground (assuming no slip). So Munkey seems to be right: one revolution of the tire has to equate to one circumference's worth of distance.

I'm afraid my radius measurement (which was easier than measuring the true circumference) was an inaccurate shortcut.

I'm about to go out into my shop. I'll try two significantly different pressures, (10 psi and 40 psi -- the tires are rated for 51 psi) and report.

Things are going downhill fast when you have to reply to your own posts, to contradict them.

Turns out, much of what I said above is bunk. I don't have to recalibrate my speedo, after all. The rear tire on this three wheeler is a scooter tire, bias ply, and the wrong shape for a vehicle that doesn't bank in turns. It's also small and narrow, while the front tires are much wider, and modern radials. This would seem to be a prescription for dramatic and dangerous oversteer, even though the front carries 60% of the weight. In fact, this is a temporary situation, because the scooter mechanicals pushing this thing down the road will disappear soon -- they are there simply to make the thing go, check overall function and suspension geometry in a general sense, and to get a couple base line MPG figures to calibrate my spreadsheet. But, getting back to the point: this vehicle has two different types of tires, and it is interesting to see how that affects tire circumference vs distance traveled per wheel rev.

On the rear tire, circumference changes a lot with inflation pressure: 58" when inflated to 5 psi, 59" when inflated to 20 psi. At 20 psi, the rolling distance, measured along my shop floor, is 57." This squares very well with the rolling radius (9 1/16"), but poorly with the 59" circumference.

The radial front tires change very little in circumference with inflation: 76" unmounted, 76 1/8 at 20 psi, 76 1/4 at 35 psi. At 20 psi,the loaded radius is 12 1/16". So the effective rolling distance should be 75 3/4. Actual travel is 76 1/4. That's fairly close to half way between Munky's prediction and Vulcan's.

So you both win! I, on the other hand, lose because my steel measuring tape (the flat 50' type) which I had for 30 years or so, and which I've always treated with reasonable care, broke not once but twice near the end as I rolled this very light vehicle over it. Seems it did not like the tread compression. The costs of science!

Vehicle speed involves engine speed, transmission reduction, axle reduction, and tire revs per mile. The errors are multiplicative - 1% on each term will yield 4% overall.

When you look at the number of gears, tire brands and sizes, plus the range of mechanical efficiencies of the drivetrain, you will find that there is substantially more error in the system than anyone would want, including those that design them!

Actually, speedometer drives have always been linked only to wheel rpm. Engine rpm has nothing at all to do with it, because the engine can be running at many different speeds for a given road speed. Likewise, transmission ratios have nothing to do with it, from the measurement perspective. Imagine the complexity of a speedometer if it had to change its internal ratios with every gear change. For these reasons, speedometer drive gears have always been on the final output shaft of the transmission*, and have always been supplied to match a particular rear end ratio. (If you changed you rear end ratio, you would buy a new speedometer drive gear.) The only variable of the ones you mention that affect speedometer accuracy is tire circumference. As you say, this can be different from brand to brand in the "same" size, and of course it varies with different sizes.

Old VW beetles had the simplest system, in which the speedo cable went right through the front spindle, and was driven by the hub (grease) cap. One wheel turn = one speedo cable turn. But in all other cars, the wheel turns/speedo cable turns ratio is also fixed, (although not 1:1) and independent of engine speed and transmission gear selection.

*Now, with ABS, there must be individual direct wheel speed sensors, so on such cars there is no need for an additional vehicle speed sensor on the transmission. I'm not aware of any car which has done away with the vehicle speed sensor, however.

Ken and Vunlcan are both Correct in their respective descriptions. Many many years ago I sold tires and the inflation and actual size of the tire have the most profound effect on the speedo.

Tire wear however will be notably higher on an uderinflated tire.

The best thing to do is to have the correct tire size to correct your speedo inflated to the proper pressure.

With metric sizing on tires it can be easily accomplished by changing the diameter of the tire by one size for every mile per hour the speedo os off. Larger tires to make the spedo read a bit slower and smaller to do the opposite.

Tires are sized like this P195/75/R15

P195 is the bead to bead measurement. /75 is the aspect ratio /r = radial 15 rim Dia.

Example: A P195/75/R15 tire sized up 1 size woould be a P205/75/R15 and will net you approximately .95mph drop on the speedo bringing it closer to actual speed as relating to say a GPS reading.

Notice that only the diameter size changes P195-P205 this is the bead to bead measurement.

You can also accomplish this effect by changing the aspect ratio which is the 75 number. to a higher number without changing the diameter.

Just a little info hope its not terribly confusing. Also it been about 28 years since I sold tires so I may have crossed up some of the info.

P195 is the bead to bead measurement. /75 is the aspect ratio /r = radial 15 rim Dia.

Unless you change wheels along with your tires, the bead to bead measurement remains the same. The "section profile width" changes. Of course, this profile width is affected by rim width, but less strongly than you might expect. Also interesting, (although not earth shaking) is the fact that the actual profile width is often different than the nominal width -- even when the tire is on the design rim width (which is usually near the middle of the acceptable rim width range). Sometimes, this difference is 5mm or so, so that e.g., some 185 section tires should really be called 190. But we don't name tires by decades.

Thanks for the add-on Ken I had forgotten to include that.

Speedo meters show vehicle speed in real time plus distance covered , that every body knows , so they are linked with wheels that actually run on roads or terrains , this is simple as well as best approach to measure velocity. engine speeds , transmissions irespective of gears have nothing to do about actual speeds , there are meters for engine speed in RPM so driver matches correct gears with speedometer reading and engine RPM for better effeciency and performance. Also circumference of wheel also matters for specific speedo , I once tested a speedo from scooter having smaller wheels with motorcycle nearly twice the diameter wheel and reading too halved for same motion speed .for calibration you require to change the heliac /screw gear ratio . Latest automobiles have digital speedos use encoder/sensor at final drive shaft , magnetic pick up or IR disk type or some bikes utilize electric techometer to signal digital LCD display

Never rely on your speedo readings, they always over read. I have a Kawasaki that at an indicated 150mph is actually doing 125mph. (clocked by the police in the Isle of Man) Don't forget, the faster you go, the higher the error. If you need better acuracy, do what others have done and get sat nav, they seem pretty good. Unless you know better.....

A friend of mine was 'done' on the Hitchen 50 mph speed limit road doing 110 mph by the police on her Triumph Sprint speed triple...

She proudly showed me the certificate as evidence, I never asked her what her speedo indicated though... !!

She only had a few points on her licence plus a hefty fine!!

Advantage of the Isle of Man....No speed limits, no ticket!

A long long time ago I took a drivers safety test to lower my insurance premium. The instructor was dealing with a lot of people in the class who were there for excess points and for speeding.

He said police will give you 8mph over the posted speed because of innacurate speedometers. To prove this he asked every speeder in the class how far over the speed they were sited for and noone was within 8 mph of the speed.

The point is that law enforcement seem to know that the vehicle speedometers are inherently wrong and showing a higher speed than you are actually going.

However I recently was driving with a gps and roadmap running off my computer and the gps speed was the same as my speedometer. Go figure.

One other minor correction.

To all from England, please don't come over to the US and ask to see someone's speedo. It would be rather unpleasant. In the US that phrase typically refers to a very minimal bathing suit for men.

Point taken! So, whats American for speedometer?

Speedometer. I don't know of a short cut phrase. I just know that speedo isn't it.

Oooo eeerrrrrr Well I didn't know that!!!

Is it a sort of thongie thing then.... or perhaps I don't really want to know?!!

Here's the wikipedia version of it. (There aren't any pictures on this page.)

http://en.wikipedia.org/wiki/Speedo

Ok folks. I come from the American Auto industry (racing) and I have always heard and said "speedo" in reference to a speedometer. Most times I would say "speedo-meter" just to be funny. Us car guys don't think of banana hammocks when some one says, "Check your speedo."

-A-

I agree -- many (most?) of us say speedo for speedometer.

Banana hammocks...

One other thing that hasn't been mentioned: The speed indicating signs that one might use on the roadway to evaluate the accuracy of one's speedometer can only be as accurate as their last calibration to within a measurable tolerance. If the calibration is to a non traceable standard, or if there has been some considerable time lapse since the calibration, the speed sign may not be accurate at all.

I personally have always believed that the most accurate way to check my speedometer was to measure the time required to travel a fixed distance on the highway as measured by 4 or 5 mile markers along the road. Use a stopwatch to clock yourself and the average the time over the distance traveled.

Off the gauge (over 240mph) on a Hyabusa is only 207mph on the GPS.

-A-

Sounds like a Ferrari speedometer!

I just finished building a small vehicle for which I needed a speedometer. I bought a bike "computer" which records current speed, max speed, avg speed, trip time, trip distance, accumulated distance, etc. It is set up by inputting your tire circumference (down to the mm), and can be tweaked, if required for under-inflated tires, etc. It can consistently read to within .5mph of actual. The pickup is magnetic and reads actual wheel speed. All for $15 US.

In the days of mechanical speedometers, driven from a gear on the transmission output shaft, 5 mph error was typical. If a speedo reads 5 mph too high at 50mph, that is not a certain indication that the odometer also reads 10% high on distance. A weak speedo needle return spring means the speed will read high, but has no effect on odometer reading.

Most cars now have vehicle speed sensors which generate pulses in proportion to wheel speed (they are transmission output shaft driven). These pulses are then used to drive an electromechanical speedo, and a fully electronic odo. The odo, at least, could be very accurate, given standard tires, and should be easy to recalibrate... although I don't know if all (any?) manufacturers make the recalibration easy.

I don't think GPS coverage is consistent enough to be used for warranty purposes -- and the cost is still high (compared to the $1.00 that an electronic odo must cost).

That's always been my belief, 'odo's (ie distance readouts) have tended to be reasonably accurate but 'speedometers' overread by design. My current car seems to overread by 5mph virtually from standing according to the mileposts; a bit more at 70mph.

I believe that today's electronic speedo can be calibrated or interfered with through OBD or CAN diagnostics.

A few weeks ago I did a web search for info regarding speedometer calibration and found a statement that the US federal regulations for automobiles requires 5% accuracy. "

As of 1997, federal standards in the United States allowed a maximum 5% error on speedometer readings (per "Auto Tutor", American Automobile Association of California magazine, Oct. 17, 1997)."

Therefore, at 30 you should read between 28.5 and 31.5 if the wheels and tires are correct. If not, and your car is within warranty, you may have a claim. However, if you try to get your dealer to fix it, you're probably going to be very frustrated. Good luck!

While tire inflation may have little effect on Tire circumference, it will have significant effect on the effective radius as measured from the axis of rotation to the road surface. It is that distance that should be employed to determine the effective circumference.

In other words tire inflation effectively changes distance traveled per rotation.

Slight tire deflation has no affect on the length of the tread. ( e.g. delta 15psi.) There are still the same number of inches all the way around the tire. Deformation does not cause the tire to "shrink." The length of the belt stays the same. The circumference does not change. Therefor, the distance traveled per rotation does not change.

People have to get their heads around the fact that, just because the tire is deformed, does not mean the circumference has changed. If you wish to challenge this, please tell me where the steel belt goes. I obviously don't understand.

-A-

Had I not just measured this stuff in the shop, I would agree completely. I repeated the checks on the scooter tire, especially, and the loaded radius gives the correct (actual distance across the floor) value, and the circumference gives the wrong value. Also, the circumference varies almost imperceptibly with the tire loaded or not (but of course the radius does vary).

My trike is atypical in the front because the tires are very lightly loaded. But their measured travel was halfway between the two possibilities. At some point, I'll have to go sit in a corner and think about it. (But the more heavily loaded -- relatively -- rear tire travel was in almost perfect agreement with the rolling radius calculation.)

It would be interesting to repeat these measurements with a new versus worn out tire in which the steel plies are almost exposed. With a new tire, you can imagine the steel ply being inextensible, in which case the tread compresses quite a lot at the contact point. With a worn out tire, the tread compression would be much less, and then the difference between the calculation methods would be less, I'd think. If the steel were in contact with the road, then I think your contention would have to be true, wouldn't it?

My destroyed tape measure probably holds the answer. There's more tread compression than we'd think. (Odd, though, that you don't get the distance back when the tread springs back... but i guess that is lost to tread scrub -- the region where the tread is expanding can't push enough to overcome the rest of the contact area, so it skids, scrubbing off tread rubber.)

I'm not convinced I really understand this. Maybe I'll experiment a little with the family car, where the differences in loaded radius are a little more dramatic. If I get really energetic I'll make a little paper-surfaced wheel with a foam rubber center -- for which your contention would have to be true, I hope.

As I said, I agree that the circumference doesn't change. It's the effective diameter that changes.

The only way I can think of proving this is to install an rotation counter on a tire and measure distance travelled per rotation on an properly inflated tire and a slightly deflated tire. Now, if someone is willing to do just that, I'd appreciate if he let us know. I'm not quite willing to do this experiment on my old clunker. If no one wants to do this, I might give Myth Busters a call.

Actually, I did that: measured distance traveled per rotation on a properly inflated tire and a significantly deflated one. On a radial, the distance traveled fell half way between the calculated distance based on effective (loaded) radius and the actual measured circumference of the tire. On a bias ply tire, the travel matched almost perfectly with the loaded radius calculation, and was 2" less than the circumference -- an amazingly large difference. So for the rear tire, your method is right on the money.

How many meters or kilometers did you measure this? It's difficult to come up with a definitive conclusion if it's only a few feet. I'm interested in putting this to rest. I need to get on with my life.

The tread scrub argument makes sense to me. I am prepared to stand corrected.

This completely jives with the fact that an improperly inflated tire will generate ALOT more heat than a properly inflated one. This fact led to a rash of tire failures across the American South a few years back.

-A-

This fact led to a rash of tire failures across the American South a few years back.

Yes. Bridgestone/Firestone and Ford were two of my best clients, by far, so that debacle was of great interest to me.

Only a few feet, namely one revolution. However I repeated this measurement about ten times, forward and backward, and the results were consistent to within 1/8". I also need to get on with my life, but remain interested, 1. because the radial/bias ply results were so different, and 2. because the speedo on my trike can be easily relcalibrated by single mm's per rotation, I should be able to get speed and distance very nearly spot on*. Before too long, I'll calibrate it against measured miles, and then experiment with tests over full miles. Then I'll post the results.

* This is a vehicle I'm entering in the X Prize competition, so accurate distance measurement is important to me.

You don't understand......!

Yeah, that does happen sometimes. One of my favorite quips is that, "I know precious little. However, I suspect a great deal."

Another point here, Andy, is that I don't mind being found wrong. I would much rather be found to be wrong than allowed to continue to BE wrong.

Maybe I'm just crazy, but that's how I roll.

-A-

Well done.

Around 1970 their was an artical in an auotomotive magazine that stated due to the design then in use a speedometer was accurate at only one speed so it was common of manufacturers to callibrate to 35 mph. It was known that below this speed it would indicate slow and above this speed it indicate fast. A 1969 Corvette with 160 mph speedometer was a good example at an indicated speed of 160 mph the acturall speed was closer to 135 mph. The calibration being performed at 35 was thought to be a happy medium that would cause the fewest number of violations due to the average speeds driven by most drivers. I hope maybe this helps clear up the logic applied to indicating a vehicles speed.