Tire size, revs/mile and the speedometer

[F8L]

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".

[gone-ot]

...short answer: yes, some tires "squish" more than others so their actual rotating "radius/diameter" under load & speed is LESS than their physical radius/diameter!

...for instance, the "common" rotating radius/diameter is 0.97 of the tires actual physical radius/diameter...but that number can (and does) vary between 0.90 and 0.99 depending upon: (a) the tire, (b) the air pressure, (c) the load, (d) the speed, etc.

[CapriRacer]

Originally Posted by F8L

I was hoping someone here could help me with a tire size question.......

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).

[F8L]

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).

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.

[royanddoreen]

changing my tires to 1.3" bigger o.d. shows a noticable difference maybe your minor change doesn't.

[gone-ot]

CapriRacer--Question: isn't the tire rev-per-mile value measured at a test speed of 45 mph and at something LESS than the tire's FULL load capacity?

[some_other_dave]

Were all of the tires new? Worn tires change diameter and revs/mile.

-soD

[CapriRacer]

Originally Posted by Old Tele man

CapriRacer--Question: isn't the tire rev-per-mile value measured at a test speed of 45 mph and at something LESS than the tire's FULL load capacity?

I don't know. I'll have to look this up.