I was hoping someone here could help me with a tire size question.
The GenII and GenIII Prius exhibit a +1.5mph to +2mph overspend reading on the speedo. When comparing the speedo to a GPS unit this overspeed reading is always there. Using an OBDII reading device like a Scangauge or Torque app shows a solid 2mph difference between the speedo and the computer reading. This part is well established.
When changing tire size to a larger or smaller diameter tire, the computer display does not change but the speedo vs. GPS rating does just as we expect it to.
So here is my question. If I install a similar sized tire as my OE tire with the a revs/mile rating of 828 vs. 829 (OE tire) why would my speedo now read accurate and agree with my GPS? The computer still reads 2mph slower.
Do some tires simply not deform as much and therefore remain larger in diameter?
The "exact" specs for the tires in question:
1. Yokohama AVID S33D (89T SL) 195/65/15 - 25" diameter, revs/mile 829
2. Michelin Energy Saver A/S (89T SL) 195/65/15 - 25" diameter, revs/mile 832
2. Continental PureContact (93V XL) 205/50/17 - 25.1" diameter, revs/mile 828
The first two tires produced the 1.5mph to 2mph speedo error that the car exhibited from the factory. The last tire is the one that got ride of the speedo error yet the revs/mile is nearly the same unless the specs put out by the manufacturers are not completely accurate. Unfortunately I did not have a chance to compare the tires side by side and properly inflated to see if there was a visual difference in diameter.
To really mix things up, I tried a set of Primacy MXM4 215/45/17 tires with a diameter of 24.7" and revs/mile of 844 and the speedo error did not increase. In fact, it was slightly reduced so it only read about 1mph too fast.
Rim width on the 15" tires was 6" and the rim width on the 17" tires was 7".