The tire pressure in your tires DOES NOT change the distance (perimeter) around the tire!!!!
( at least not by any measureable amount, so for all practical purposes, stays the same )

Say the perimeter of the tire is normally 2000mm. Flat = 1999.9mm. 45psi = 2000mm.
Only the shape changes with pressure changes.
The more air pressure you have, the closer to a circle with 2000mm circumference you have. Less air = a longer flat spot on the bottom.
No matter what pressure you have, the distance around the tire, and your MPG or odometer is NOT affected.
( As the tire tread wears off, your tires will get slightly smaller. )

-John


P.S. It may help to visualize your tire like a tank's tread. No matter how many rollers the tank has, or the shape of the tank, the tank tread length never changes to make one revolution. The distance traveled for your tire tread to make one revolution NEVER changes.

You're right of course, John, though it's counter-intuitive.

Thanks,

Colin

That does make the presumption that the tread on a tire cannot stretch... which, now after I've thought about it, is likely true since all tires are steel belted these days. I doubt the steel belt would have much give in it at all.

Lacking the steel belt the rubber could stretch SOME.... but probably not much since the tread rubber is pretty thick. Bike tires for example definitely have different circumferences depending on inflation pressure, which is why you have to adjust the speedometers for distance if they use a magnet to count tire revolutions (it says that in the manual I have for mine). But of course Bike tires aren't steel belted and are a heck of a lot thinner.

I hadn't thought of the steel belt when I posted and was thinking of that bike speedometer at the time.

I have the NAV radio, and I noticed that the volume will "blast" quite loud, for a split second, when I am going very slow. For example, when I am pulling into the garage, and I am about 1/2 in, the radio's sound will "blast" and then return back to the expected volume level. It will also do this, sometimes, just as I am just about to come to a full stop at a red light or signal.

From your posting, I wonder if this is because of the AVC (Automatic Volume Control) setting? I had been thinking it was simply a bug in the firmware which caused the momentary "blast".

Best single LD trip: 35.3mpg

It is... I have set mine very low because I've experienced the same blast. If you set the AVC all the way to the left you'll find the blast effect goes away. Actually, it still is a firmware issue in the sense that whomever programmed the darn thing should have made it use a running average of speed rather than the instantaneous speed it appears to be using. Apprently it gets a spike in the readings sometimes and it responds and blows your ears off.

That does make the presumption that the tread on a tire cannot stretch

That was my mistake. I was thinking of the extreme case of Top Fuel dragsters. They use non-steel belted tires inflated to around 6psi and the centrifugal force greatly increases the tire O/D at high revs, acting rather like a CVT.

Compare stationary to rotating:



Definately a change in diameter.

Sorry it's completely off-topic, I'll shut up now.

Colin

If this is indeed true, then how do many manufacturers (including some Ford models) utilize the ABS sensors instead of actual air pressure sensors to operate their TPMS systems?

For example, this was taken from from the 2002 Windstar shop manual section 204-04:

"The Low Tire Warning (LTW) system detects differences in inflation pressures in one or more tires. The system uses the ABS wheel speed sensors to monitor the rolling radius of the wheel and tire assemblies."


A few other vehicles that use this approach include 2001 to 2003 Ford Windstar, 1997-2002 Buick Park Avenue, 1999-2003 Buick Century & Regal, 2002-2003 Buick LeSabre, 2003 Buick Rendezvous & Aztek, 2000-2003 Chevy Impala & Monte Carlo, 1999-2002 Chevy Alero, 1999-2002 Pontiac Grand Am, 1997 - 2003 Pontiac Grand Prix, 2000 - 2003 Pontiac Bonneville, and 2001-2003 Oldsmobile Aurora. Source: http://www.aa1car.com/library/tire_monitors.htm

So maybe I was right and I gave up too soon.

Its been rolling around in my head for a while now. Even with a steel belt the MIDDLE of the flat belt can likely bow in or out changing the diameter of the tire slightly.

As for the tank tread comparison.... um.. tank treads aren't Pnuematic. Clearly as you fill a balloon with air the diameter around the ballon increases. A rubber tire is like a ballon except for the fact that its thicker and has a steel belt. from the info above it appears that even a steel belt will give some. 40PSI provides a significant amount of force when you multiply 40 times the area of the inside of the tire in square inches! All that force is pushing outward against the tire rubber trying to maintain shape.

If you can measure (even inaccurately) tire pressures using the speed of rotation, the distance around is not constant with air pressure.

TeeSter, I have to agree with you.

The tank tread analogy works well in a theoretical/perfect world, but it's nowhere near accurate when other variables (such as the rubber compound expanding, compressing, and slipping against the road surface - among others) are introduced in real world conditions.

Anyone that's every driven a 4WD truck with a locking front axle knows only too well how even a minor side-to-side variation in air pressure will induce a major pull to the side with lower pressure.

Don't NASCAR crews alter their air pressures left side to right side to make the inner wheels a slightly smaller diameter so the car turns easier as well?