I know that a lot of people here run their tires a few PSI over the recommended levels. I was just wondering if anyone had ever tested whether or not this changes the rolling circumference and hence the accuracy of the speedometer and odometer readings?
I seriously doubt you would read a difference. The circumference changes so little, hardly worth mentioning.
Uart, I agree with Charles; the change in circumference based on two, three, four, or five pound air pressure would, in the most likely, not register as a change in circumference. Besides that, I have driven through a couple of stationary radar display stations, several different times. My dash reading is consistently two MHP faster than the readout returned on the radar unit. That seems like a worse error percentage than I would get by adding a few pounds of pressure. --TK
Yeah I agree that you wouldn't expect much change. I was just inflating my tires yesterday (to 42/40) and was pondering why higher pressure gives lower rolling resistance. I figured it must be because the tire deflects less, so less deformation gives fewer losses, but also a smaller contact patch and perhaps a very slightly larger roll out distance. Anyway I thought it would probably be negligible but was just wondering if anyone had ever tried measuring it. (I figured if anyone on Earth had gone to the bother of testing it then it would probably be someone here, hehe ).
I don't think so. I'm running 42/40 and my GPS reports 1 MPH under the speedo. EDIT: This is at highway speeds - 55 to 70. I haven't looked at it at lower speeds.
Ok so both yourself and TKY have reported that your Prius speedo's read slightly fast. That means that even if there where any small increase in the roll out distance, which would make the speedo read just a little slower, then it would probably make it more accurate (since it's currently reading a little over speed). That's good to know.
My opinion, too. Try pushing a car with a near flat tire, then try pushing it with the tires inflated. HUGE difference. So pressure really does reduce rolling resistance. BTW, I don't know why but my speedo is really close to right on, maybe off 1 MPH @ 70 (which would be a half MPH @ 35).
A bigger change in circumferrence would probably be seen between new and worn tires than between inflated a few psi different. IMHO, anyway.
The circumference of a radial belted tire does not change with pressure. Therefor the speedometer and odometer calibration is not affected. Tom
The circumference change appears to be smaller than the indicated mileage impact. The effective circumference should change slightly, that is an important aspect of the mpg improvement: reduced friction losses from sidewall deformation. The best way to test the circumference effect is probably to do an odometer test over a fixed distance with multiple tire pressures, including repeats in each direction.
