Tire pressure affect on tire size

[SPLThunder]

I am trying to figure out if change in tire pressure could account for a 0.2-0.5 mile discrepancy over the course of a 30 mile test.

I am basically wondering how much (in %) total distance traveled could change as a function of tire pressure. Or how much (in %) the tire size (circumference) could change as a function of tire pressure.

[vgames33]

I don't think it really matters. A tire of outside diameter X" should always have X*3.14" of contact distance.

[slideways...]

if the tire were made of aluminum, this would be true. but spl is right in that lower tire pressure will give an effectively smaller rolling radius.
figure out how many revolutions each tire makes in a mile. then let some air out and re measure the circumference using the new tire radius from the hub center to the contact patch, then recalculate.

[vgames33]

But for each revolution, all of the tread will still have to rotate and contact the ground. The length of the tread won't change due to a loss of air. The shape of the tire will just deform more when weight is applied.

[MagicRat]

Originally Posted by vgames33

But for each revolution, all of the tread will still have to rotate and contact the ground. The length of the tread won't change due to a loss of air. The shape of the tire will just deform more when weight is applied.

Good point. I would agree, unless the tire was severely underinflated, the difference would be marginal.

[slideways...]

Originally Posted by MagicRat

Good point. I would agree, unless the tire was severely underinflated, the difference would be marginal.

true. the effect would be small but still present. the lower the tire pressure the closer the road is to the center of the rim, and that distance is the rolling radius. doesnt matter about tread that isnt touching anything.

[curtis73]

Its true; the outside circumference of the tire will remain constant, but there might be more or less scuffing with different tire pressures.

[Nereth]

For a radial tyre, it is pretty insignificant, although there will be a difference. Bias-plys are another matter.

I wouldn't worry about such a small discrepency. Maybe you changed lanes a few more times one way, or maybe you were a bit more lead footed.

[UncleBob]

before they started putting pressure sensors on the inside of the rims, the low tire pressure systems that were used previously watched the wheel sensor for the ABS, and if it noted any change in the speed of rotation vs the other tires, it would set the tire light

There is SOME change in diameter, depending on tire pressures. Thats why a car will pull if you deflate one front tire and/or overinflate the other front tire

[bobss396]

You would have to really inflate a tire to an unsafe pressure to get more circumference out of it. It will be measurably larger.

You could do some backward calculations on a speed chart to determine the size delta that you would need to achieve your numbers.

We used to stretch stock car tires to get a desired "stagger set" size if we needed it. Not at all recommended, but we would inflate the tire to around 60 psi and leave it in the sun all day. Right before we would use it, the air pressure would be brought back to normal. We would pick up around .5 to 1 inch of circumference. Precautions were taken to lessen chances of personal injury.

Bob

[j cAT]

Originally Posted by SPLThunder

I am trying to figure out if change in tire pressure could account for a 0.2-0.5 mile discrepancy over the course of a 30 mile test.

I am basically wondering how much (in %) total distance traveled could change as a function of tire pressure. Or how much (in %) the tire size (circumference) could change as a function of tire pressure.

if you fill tires to max allowed pressure you will have minimum rolling resistance this then would reduce slippage in the transmission, which could account for some difference in the recorded mileage....tires are like ballons more air the bigger they get... also if it where 20deg f or 110deg f you would see a difference in miles traveled......with the change in temps......interesting observation............