Speedometer Accuracy

That's about what mine shows. I think vehicle manufacturers err on the side of safety. Had you rather it show 63 when you are doing 68?

There was a very long and boring discussion on GPS accuracy here some time ago. GPS is very accurate, but I'd go with my speedometer. One less thing to distract you while driving.

Because you are in the car you are in your own inertial frame of reference and thus subject to time dilation relative to the Earth and it's geostationary satellites.
C'mon. just stop and think about it... a few extra psi in your tyres will effect your true speed compared to the speedo reading.
3.1% seems pretty good to me... if only I could get that much interest on my savings.
How about the situation in North Africa, the state of the economy, climate change, the fate of the whale... maybe they are worth worrying about.
But a 3% error in your speedo which you can't do anything to adjust or recalibrate?

I don't think you will get a ticket for being 3% over thespeed limit.
Or maybe you are just bored?

Ouch! Just curious!!

Check your tread depth.

It is easy to calibrate the speedometer if you spend any time on the Interstate system. Just time your passage of the posted mileage markers with a digital watch which includes a stopwatch while you are driving at a preselected speed. On a several hour trip you can get significant accuracy (below 0.1 mph) over a range of speeds. It is up to you how involved you are in the calibrating function- this is an engineering website, after all.

The GPS is accurate if you ignore sporadic fluctuations.

My wife's 2010 Impresa says 110km/h when we are doing 100km/h. Speedos vary all over the place but always on the side of reading high.

You can calibrate the speedometer for more accurate reading. Small % of difference in reading will not create problem. GPS is more accurate than speedometer.

As a general rule of thumb your GPS will be significantly more accurate than the speedometer in the average car. Where you will find a difference is when you are travelling on a winding road, your speedometer is measuring direct road speed where as your GPS will calculate the average speed over the net distance travelled (as the crow flys). Because of this the GPS may read slower than your actual road speed. On a dead straight road you can rely on the GPS to be more accurate for a given distance.

I use my gps as a reference as I drive several different cars (vehicles) and I've noticed some speedo's are better calibrated than others.

Tyre sizes will play a significant role in determining the accuracy of your speedometer. I changed back to 14"x70 profile tyres from 15" x 65 on one of my cars and now my speedo is roughly 10% slow i.e. shows 100kmh and is really 92kmh. No biggy but as my other car is similar (one is a sedan the other a wagon of same make/model)I use the GPS to keep my reference as otherwise Cunstable plod will have me good n proper.. so to speak.

The Australian Design Rules (there are similar regulations in the US/UK/Europe) allows for -10% deviation in accuracy that is it is allowed to show you a higher reading than what you are doing. Most manufacturers will err on the side of caution.

Where the discrepancy will really get you is the odometer. Most odometers are reasonably accurate in comparison. However 10% faster reading on the odometer will artificially age your car. Not so bad for low milage urban dwellers but guys like me that do serious milage in a year it is noticeable.

There are automotive instrument technicians that can make the recalibration for you, however when you change your tyres you'll have to get it adjusted again.

Calibration of the speedo system is simple in theory but relatively complex in practice. It is a function of:

• Engine speed and accuracy (and repeatability) in reading it,

• Transmission ratio,
• Rear axle ratio, and
• Tire revs/mile (which is probably the least accurate of the variables)

They work in the formula:

MPH = (engine revs/minute)(60 minutes/hour) / [(Trans ratio)(Axle ratio)(Tire revs/mile)]

You can see that all errors are multiplicative. If you start playing with the percentages you will see that various low percentages of error in the components (+/- 1 or 2 percent) will lead up to more error than you would think possible!

I do not know, a speedometer seems accurate than gps for me. Commercialized GPS goes with standard preset +/-10 m accuracy for National Safety reasons.

Knowing that speedometer is incomparable.

+/- 10m of absolute position is quite different from changes in relative position. On top of that depending on the algorithm the error in change in relative position on velocity becomes diminishing low. The one thing accurate in GPS is the clock, so the velocity accuracy on changes in relative position over sufficient distance is extremely accurate.

These techniques are similar to velocity feedback from absolute shaft position encoders when the actual absolute position feedback can be +/- 1 degree in shaft position, but over 10 revolutions the error is still the same on any given reading but the total accumulated is only 360 x 10 deg +/- 1 degree, the more revolutions, the better the velocity feedback. As an example calculate the speed with an accurate clock over 1 second for a shaft at 1200 rpm -- there are 20 x 360 degrees and the error of 1 degree gives a velocity error of only .014%.

The same reasoning gives very accurate velocity readings on the straightaway for a GPS.

I supposed, this computation you got here for velocity(GPS), compose, radius of measurements(elevation) -computation by polar coordinates. Looking above on reference a satellite down below is just a plane, any movements of frames gives you a rate with respect to that plane(x, y), however the real thing is, at surface of the earth, its not all plane(x,y,z), how about downhill & uphill road curves. Definitely, by just looking on the moving car above, you will not notice the curves along which the car is moving.

Not, unless I read the GPS manuals, I will not be convinced.

It's a pleasure if you will show me some illustration GW

"I know vehicle speedometers are not always perfect"..

Police said true..

There is none in this world having error "0"

Well, hi folks, with you again. There is no need to tell somebody he is bored, nor to relate every little bit of life to North or South Africa, or this or that. I personally noticed this speedo tendency to show some more speed than actually present. Annoying at the beginning, because if the auto guys can brew such accurate powertrains and engines, why not be more precise with the speedo? But, still, this is what they do. If it helps reduce accidents and increase responsibility? I dunno, and maybe somebody should tell us what is the situation with a Ferrari or a Lambo: do they also show less when one buys them for more?

There is some concern about "liability" that you have alluded to.

When it comes to "bespoke" cars like Ferrari, Lambo, et al then the tolerances are somewhat closer as the "embarrassment" factor for showing higher speed than actual is significant.

Cooking or garden variety mass market cars are slightly different where class actions for being booked for speeding are a reality in litigious societies. So to err on the "safe" side makes sense for the bean counters.

As I mentioned in my previous post the various transport department, have rules that allow for speedometers to be inaccurate in that way.

How often does a GPS recompute your position? For instance, if you travel one way for 10 seconds at 20mph, and then turn back for 10 seconds at 20mph, your net movement will be zero. If the GPS just happens to take these same 20 seconds to update, it will compute zero mph, even though your speed was mostly 20 mph. And similar but lesser errors on curvy roads.

I suspect that people are trying to use their GPSs to welch out of speeding tickets, but they don't know any damn thing about geometry....

True

And they depend for real accuracy, on how many satellites are in view.

Commonly they can be several hundred meters out if you are moving, so 'velocity' is not necessarily 'accurate' but a 'guesstimate' on trend against a straight line/heading.

Obviously this doesn't matter a lot in aviation or marine use, but in such as 'where is the turnoff'? in a rural context - it tends to make one aware of the 'limitations'.

I have never been able to reverse directions, run extremely curvy roads, or other maneuvers that would upset a GPS and be speeding at the same time.

When I am in school zones, watching a GPS to see if it is correct is the last thing on my mind. I am watching for kids running out from between parked cars or running for the bus, horseplay beside the road, etc.

When I speed it is on the free way with straightaways and long curving bends, and even then I try not to be the fastest. I am quite content to let the young buck in his fast car keep the consatbles busy. In this situation it is easy to check the GPS.

Missing intersections out in the sticks, yes, but I believe that is a map error, since city maps appear to be very accurate.

It would be to the auto manufacturer's advantage to have the speedometer and the odometer read more than they actually sense. In this way the owner will take the car in for an oil change or other maintenance at 2-3% of the mileage earlier than reality. i.e. at 2940 miles instead of 3000. In this way the manufacturer gets more oil changes earlier than with an accurate speedometer/odometer.

I just did the math on this. Over 100k miles, a car reading 2% lower would show around 2000 miles difference. Not really enough to make this theory stick.

I've compared four different vehicles with a GPS. My Corolla matches almost exactly. I tried a Windstar, a Ranger pickup and a Subaru Outback that all read lower than the GPS. Confirms that I have to be careful when I go through known speed traps in the Corolla; I've got no breathing room.

On a fairly straight road, the GPS will be the most accurate, usually less thean +/- 1MPH, probably even on less than straight roads too!!

Spedometers are only accurate at one tyre pressure with a certain depth of tread on a certain tyre type, with one specific load - maybe! eg. not accurate at all...

The law in most countries requires that a speedo NEVER shows a slower speed than is being driven, but it may show a higher speed than actual.....many years ago it was minus nothing or plus 20%. I do believe its only plus 10% or so now.....but don't quote me....

I find that Volvos and Mercs are generally speaking only about 1-2 % fast, whereas most other cars are generally around 5% fast....I drive around 5 or 6 different cars for a friend of mine each week.....Volvos, BMWs, Audis and VWs usually.....

I too have noticed about a 1-3 mph optimistic reading on the speedo at highway speeds when compared to GPS on straight, relatively flat roadways. One interesting thing to note, when I viewed the vehicle speed data off of my ODB-II monitor, it was dead nuts on with the GPS. The vehicle computer knows your true speed even if the human readable gauge is a little off.

BTW, tire inflation makes very little difference (if at all) unless you are scrubbing the hell out the tread or spinning the tire bead on the rim. Think about it.

You gave me some food for thought, so I played around with some figures to see what differences might be legally achieved, here is what I came up with.....

Tyre pressure and load change the distance from the center of the wheel to the road, thats the flat spot on the tyre. It can easily exceed 1" from "down" to "up"...and thats probably within the "legal" bounds of allowed min/max tyre pressure and min/max vehicle loading!

Depending upon its size, two things happen, if its big (low tyre pressure, high vehicle load), the diameter is effectively reduced. Also the tyre "works" too hard and heats up, sometimes to dangerous levels, even at "legal" pressures, as many people find out each year when a "suddenly" tyre explodes......its been a "hobby" of mine for over 40 years to always run with the maximum legal tyre pressure. That has kept me safe......

I don't have a car to hand at this moment to measure, but let's assume the radius to the ground is 12.25" lightly loaded. Let us assume for the moment that the radius to the ground changed by 1" between loaded and unloaded, with low and high tyre pressure. This implies a diameter change of 2". (although the top of the tyre will of course not be flattened, but it plays no role here....

Then using the following formula C = pi * d will give us the circumference for each diameter.

C= pi * 23" = 72.3"

C= pi * 25" = 78.6"

A difference of 6.3" = (6.3/72.3) * 100 = 8.7% Difference, which would also be the percentage of speed difference for the same revs of the driving wheels......

Its a large enough difference here for example to get you a speeding ticket or not as we are restricted to around 5% overspeed before it gets expensive!!

So I personally see such a difference to be valid, though from high to low would actually slow the car down of course.......so it would be fine if you were watching the speedo carefully. But if you were unloaded, with high pressures and also running over the allowed speed anyway, it might make a difference between a ticket or not!!

A 1% difference we could safely ignore, but not more than that......

Each to his own ideas, but I was somewhat surprised to see just what such a difference tyre pressure and load can make.......even half that could still be significant difference if driving faster than allowed........lets say you get 5% pay rise suddenly, would you regard that as significant? I know I did.......

Have a great day.

In spite of the radius change as the pressure drops, the circumference of the tire doesn't change. For a given number of revolutions, the tire will travel a fixed distance unless we are scrubbing the tire. That is the conundrum of this problem.

You do change the available torque however.

Cheers !

Sorry, I simply don't believe you. Try writing a proof that I might understand....

My point is:-

If we let a tyre down (or both of them on the same axle if a 4 wheeled vehicle - not 4WD!!) to allow the tyre to sink to say half its thickness and turn the wheels, without scrubbing or anything say 10 revolutions, it will not travel as far as when fully inflated......torque will (as you pointed out) also increased for the same amount of energy put in, as you have effectively gone to a lower gear.....the extra torque must come from somewhere....

But tire wear definitely changes the diameter!

You may find that a new vehicle, with specified new tyres, [inflated to correct tyre manufactures pressure for the anticipated average payload], minimum fuel, unladen, shows a close to correct speed reading. Alterations to the above then produce amounts of 'optimism'.

As will fitting 'other tyres' with different profiles.

On set of snow and mud tyres and rims I use on my 4WD, though offered on a different model of the same vehicle, produce 10% 'pessimistic'.

The dealer fitted 'sexy tyres' produced 5% 'optimistic'. Another set of tyres 10% 'optimistic'. All makes for a lot of 'sums' when 'near naughty' speeds.

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And just a comment on the 'maps' in rural areas having some effect on GPS; the situation I exampled is overshooting an issued GPS defined lat-long, and is totally attributable to 'number of satellites' and 'refresh rate' while moving. If you have less than 5 satellites - they are 'approximating'

Cryptic enough?

That's cute! I once made a class demo of elliptical gears out of jigsawed and filed Plexiglas. Not nearly as precise as this!

I like those gears!

Tire wear still changes the diameter.

Pressure also if sufficient to stretch the wheel to a larger diameter.

Rubber compression at the nip point has its own properties. In closing a rubber (or compliant) coated roll with a hard steel roll I have noted there is an RPM difference that occurs between the rolls as the force changes. Small, nonlinear, but measurable.

An indicated 65mph on a slope of cos^-1(63/65) to local horizontal will correlate with 63mph on a GPS were the two to be accurately calibrated.

Speedometer accuracy is dependent on tyre pressure, tyre overall wear and vehicle loading.

Perfectly put, the same as I wrote a few days ago.........

We do a lot of that here...

LOL

Its a race to see who gets there first!!!

25% grade?

max highway grade around here is 7% and that gives cos^-1(64.84/65)

[or sqrt(100^2-7^2)x 65/100 = 64.84mph indicated on 65 actual]

And that assumes the GPS does not compensate for elevation changes, which may be a reasonable assumption.

Surely the law recognises the speedo and not GPS; therefore GPS should only be academic.

Other threads have included going up and down hills where distance traveled horizontally may not be as great as including the vertical axis

Tony

Well actually 'the law' only recognizes what the "Police reported" as evaluated against the "policing protocols", the Police employ in policing law. Which is the function Police are named after.