Does Speed/Distance Indication on Odometer Change With Tire Inflation

Yep. It is true.

However, it also varies with the terrain. Mileage as recorded by the odometer will be higher if the vehicle is spending a lot of the journey going up and down hills. Think about it triangularly.

Thanks for the reply , any idea what would be the variation 10% or more ?

Best Regards,

George Kottackakathu

The difference between under- and over-inflated tyres, by the sound of it.

George,

The older type odometer counted the number of wheel revolutions. They have two error factors:

1. The rolling radius of the tyre varies with inflation AND speed, the tyre grows as you go faster - look at drag cars at take off for an extreme example plus the obvious one that the more pressure the harder the tyre is the bigger diameter it has. hence underinflated tyres will show a higher odometer reading as the tyre needs to turn more for the same distance.

2. The slip factor of the tyre. This also is twofold, the rubbers grip is imperfect and the tyre slips a little each turn, so the tyre turns more or less than the actual vehicle speed. Even the Supersonic car Thrust SSC's wheels turned less than actual speed and they weren't driven and had a ten ton loading! The second element is the steering geometry introduces an amount of slip, more or less due to the tyres not tracking the same line as the car (obviously the tyres turn first and the car follows to put it in simple terms)

Both these factors add together to give you your 10% loss, of course underinflated tyres change the steering geometry AND the contact patch area as well as the tyres rolling radius so it is not a simple sum

hope this is helpful

Check this old thread on the same subject

http://cr4.globalspec.com/thread/10425#newcomments

regards,

Vulcan

Hello Vulcan,

Thanks for the thread, the link you send is very useful and going through it. Nice to understand that alot of fruitful discussion has been made in this regard.

Thanks for the link.

Best regards,
George kottackakathu

Tyre pressure will impact your Odometer readings and anything the onboard computer includes this in calculations for, such as fuel efficiency.

Do not foget however that the GPS system accuracy varies as well.

These variations come from the number of satellites you get a signal from, the age of your GPS and what the Owners of the satellite have setup as thier current variation.

This system has a military origin and there are deliberate inaccuracies built into the system which can be turned on or off at any time.

Regards,
Sapper

Hi Sapper,

Thanks for the reply.

Thanks & regards,

George Kottackakathu

Absolutely not! Who ever came up with this needs to be questioned weather or not they are able to function in an Engineering position. The tire has only one circumference, inflated or not. Think about this: what if it was possible! what do you think would happen to a vehicle racing on a track and it's stability? Because tire pressure increases on a hot surface (One of the reason they use:Nitrogen) But again No! it is absolutely Not Possible, Period.

Get a grip. This is the real world, not a textbook problem. As an integral part of the vehicle's suspension system, pneumatic tires are designed to be flexible. The weight of the vehicle deforms the circumference where the tread contacts the road surface. The pressure of the air inside the tire resists that deformation. Less pressure = more deformation.

Or are you saying that this would still have that big flat place on the bottom if it was properly inflated?

Agree 100%. Although it will be much harder to move the (example 100" of tire tread) if it is under inflated, it still must be rotated to achieve movement.

Guest,

Why not try it? Deflate your tires by a certain amount (enough to be noticable) and drive a kilometer or so and note your distance travelled. Then turn around, reinflate your tires and drive back the same way.

This was tried before in the previous thread and proved to be true for certain tires. Under inflated tires do affect the odometer and speedometer readings.

Regards,

Vulcan

You wrote: The tire has only one circumference,

Let's take a look at geometry: circumference=2πr=6.28r Agree? The car's speedometer is designed for a given circumference. As long as you don't change it, it will give you a fairly accurate reading

You are right by saying that the speed reading depends on the circumference of the wheel, But surely you know that the speedometer actually measures the wheel's RPM.

the problem is that when the tire (tyre) is inflated or deflated, the radius "r" is changing causing the circumference to change with it, hence to change the number of wheel revolutions /distance travelled. causing finally a different reading of speed. Smaller "r" will results higher reading higher "r" will give lower reading. The vertical radius is different from the horizontal radius. and that's where the confusion lays. The only time when reading will Not change with tire situation is when the speedometer will read angular velocity translated to linear velocity Some advanced cars speed measuring do just that.

As far as GPS is concerned, Here the measurements are time difference between two points, translated to distance/time, and the effect of the wheel shape or size does not play any role.

Wangito.

I am interested to understand the logic behind some of the replies posted. It seems to me that the only way a 10% difference in odoometer reading could be mpacted by tire pressure is if the tire pressure modified the circumference of the tire by 10%.

Isn't it?But on the other hand, do they think only stretching the rubber tire a 10% you get a 10% change in the cicunference?I think teorically both sides are wrong but the final answer needs experimentation.-

Most speedometers/odometers have a factory error of up to 10%.

I would say yes, that tires with lower pressure will get less range/milage then tires at proper pressure.

I also know that the ammount of tread left on your tire will affect this as well.

On my motorcycle I have changed the gearing to get more low end power. I also added an electronic component (Speedohealer V4) to correct the difference (almost 20%) in gearing changes. I calibrated the speedometer (using a GPS) when I had tires with 40% tread remaining. When I installed new tires it also changed my speedometer accuracy.

To the Guest poster posting in bold: Tires with low pressure will have a different circumference then tires with a higher pressure. You say the circumference can not change, but it does.

No your speedometer/odometer readings come from you transition, not your tires.

I don't think this is the case in all vehicles.

My Speedo/Odometer is driven by the crank speed and not a sensor on tires, rims etc.

Yup, Most speedo\odometers have thier driver in the trasmission output.

So any variable after that point like tyre wear, tyre pressure or modifications to the rims and running gear will all change the baseline value and must be catered for.

If for example we ignore every thing except tyre wear.

The tread depth 10mm for simplicity. Radius is 500mm when new, again for simplicity.
So I drive really badly and grind away 5mm.

New = 2*3.1415926*500mm = 3.14meters travelled for each rotation

Worn 5mm tread = 2*3.1415926*495mm = 3.11meters travelled for each rotation

That single small change is 1% change. Now if we allow for a 20mm change in radius due to air pressure

Worn 5mm tread + 20mm for air pressure change
= 2*3.1415926*475mm = 2.98meters travelled for each rotation

Now we have 5% variation.

As previously noted by others there are many other factors each one changes the baseline figure, which is why the manufacturers allow a 10% margin in Odometer accuracy.

Regads,
Sapper

Simply put: not appreciably. But...this appears to be more a "textbook" question.

Adding to the complexity, I have a few thoughts to inject.

Does the vehicle weigh 3400 lbs or 9500 lbs.

Is it traveling 55 mph or 80 mph. What brand of tire and how many plys?

On a top fuel dragster or a Funny car, the tire acts as a variable transmission all the way down the 1/4 mile track as the tires change diameter dramatically. It is designed to do just that.

Do a 4 ply and a 6 ply passenger vehicle tire grow much at "normal" highway speeds from centrifugal force? I know that they do............. they absolutely have to, but at speeds like 60 mph, just how much do they grow at the actual point where the rubber meets the road? I am guessing not a large amount.

Would a 4 ply Bridgestone tire behave the same as a 4 ply Goodyear tire of the same basic design?

Interesting question, but need a lot of input to come out with a generally agree'd upon percentage of error.

I just joined, so pardon my response if it seems to deep for the intended topic's initial question. I always have more questions than answers.

Lets go back to racing. Tire pressure is determined by patch contact. A pyrometer or thermal viewer are used to check for even heat across the tire. Over inflated the center strip is hotter (it sicks up higher than the side-walls). Under inflated the outer edges are hotter( the center is being pushed in and the side-walls exert more pressure on the road/tarmac). Proper inflation the heat is uniform across the tire.

I used to set my back tire pressure by spinning my wheels and checking my burn out for an even mark across the tread left on the pavement( back then a pyrometer was expensive and my tires cost me $45 a set mounted and balanced)

To test how much the difference each tire changes in size for the pressure just measure the circumference at say 15lbs psi to 5 over max psi, off the vehicle, in 5lb increments. Now graph your results and extrapolate for higher pressures. I don't recommend going higher without a tire cage because some tires are not very tolerant of over pressure.

Brad

this is all great discussion, but as a tire tech in the real world... pretty near all passenger vehicals run between 28-35 psi - if you inflate one tire to 28 psi on the front of your car, and one to 35 psi - you'll notice a bit of a pull to the under inflated tire... but 10%? i don't freakin think so... with tire inflation alone, i wouldn't think you could get more than a 2% difference under NORMAL conditions... I would check to see that the correct tire size is installed first, then check tire pressures... - ill test this on my way to work next time - ill record the distance to work - then lower pressures and record the distance home - i highly doubt i will see anymore than a 2% max difference -

Odd I haven't run 35psi in years, 45-65psi 4 to 8 ply on a pickup.

I' think you will find your bead to bead size and rim width of the tire will have a large part to play on the % of change.

Brad

that's why i said "passenger vehicals" - not "light truck vehicals" :p

even still - an LT tire is probably even less likely to be affected by tire pressure change...

Back to the original question........

"I am confronted with a postulation that based on the tire inflation levels the speed/ distance indication on the odometer changes by up to 10 % in the reading, is it true?."

I wonder not only about tire pressure, but the speed of the tire rotation and when and where the original tire diameter was measured.

Was the tire diameter measured vertically with the vehicle weight bearing on it?

How does the diameter or radius change with relation to vehicle weight and centrifugal force of this rotating mass at 45 or at 75 MPH? ..................probably not a lot, but it certainly must vary.

I know I'm nit pickin', but the tire radius at the vertical contact point (hub center to road surface), has to grow unless there are vehicle downforces counteracting this effect.

(I'm trying to think like an engineer. ;) )

In my experience with vehicles, the answer is NO! However, having said that, you will find a error in most speedos/odos. Since 1969, I think I've seen one reference to an accurate speedo, an Alfa 147 I think.

Most manufactures will change the speedo cable/pulse generator gear (on gearbox extension) for differing final drive ratios but don't for differing tyre/wheel combinations available on the vehicle.

For example, the 1998 Mitsubishi Magna (Aussie) came with 205x65x15 tyres in the base models, 215x60x15 for the Verada and 215x55x17 for the VRS. The base models' speedo reads 8% fast and the VRS reads 4% fast. The law allows an increase of 15mm in overall diameter which would be roughly a 2.5% increase in dia. Therefore the VRS could legally have a rolling dia of 683.3mm or 11% greater than the standard model.

The makers are covering their arses by not allowing slow reading speedos on the roads.

Add this to the fact that nearly all speedos read fast for safety and the fact that in Australia, speedos only have to be accurate to 10%.

If you have an electronic speedo, there is a kit in Australia for about AU$80.00 which intercepts the pulses and effectively recalibrates the speedo.

RRV