Quote:
Originally Posted by CapriRacer
First is that the RPM (rev/mile) values are from a standard condition - some load and inflation pressure. If you are using a different load or inflation pressure, then you would get slightly different RPM's. That's the way indirect TPMS's work to determine a low pressure.
Will the RPM's be the same for different tire sizes? Probably not.
So what Tele man said is true. - and I would add that this likely varies a bit from manufacturer to manufacturer.
I also note that the 2 195/65R15's are both T speed rated, while the 205/50R17 is V rated. That probably means the T rated tires do not have cap plies (circumferetial nylon overlays), while the V rated tire would probably have 2. Cap plies restrict the growth of the tire - and 2 would restrict it even more. If you are using elevated inflation pressures, it is likely that the T rated tires have a larger diameter. In other words, they are bulging out due to the increased pressure, while the V rated tire isn't (much).
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Thank you for your input, CapriRacer.
I thought about the bulging possibility but wouldn't that mean that the T-rated tires would grow larger and thus reduce the overspeed reading whereas the V rated tire would remain constant? What I am observing is that the T rated (OEM) tires are creating an error on the speedometer. The speedo consistently reads 1.5mph-2mph faster than the GPS says I am traveling. The V rated 205/50/17 XL tire does not do this. Keep in mind this overspeed rating is inherent to all 2004-2012 Prius models with 15" wheels.
Inflation pressures didn't seem to affect much in this case. I have ran between 34psi and 44psi and the error is consistent. I'm currently running 36/34psi in the 17" tires.