Do all 951's run underdrive using recommended rear tire size?

wild man · 03-20-2010, 08:47 AM

After seeing the 951 tire thread (not tread) rise up to the surface again, where I had mentioned this situation, I am now posing a question. One time when I decided to check the speed-o/ode-o accuracy of my 951 with standard 245/45-VR16 size tires on the rear(al-be-it, pretty low on tread depth), it came up short. It was short by a factor of putting an extra 7 miles on it for every 100 traveled. And that of course would mean speed indication would be off by 7%, also. Is anyone eles's 951 off by this much?

Ga 951 · 03-20-2010, 08:12 PM

Local Sheriff has had some of those roadside radar warning signs up for the last few days. Mine has been right on every time. rear 225.50.16 front 205.55.16

wild man · 05:33 AM

Are the sizes you listed the recommended sizes for a non S 951?

And what about odometer error? You need to check odometer error against the yard sticks!

Lawrence Coppari · 03-22-2010, 10:13 AM

Mine appears correct with 225/60-16 rear tires. These tires are for normal driving. Track tires, R compound, are 225/50-16. I had about a 7% error with the 225/50-16 on the rear according to other vehicles I have as well as radar speed indicators.

jkb58 · 03-22-2010, 10:29 AM

Not uncommon for the speedometer to be somewhat optimistic. Indicating even up to 10% over actual speed is not unheard of with the 944 VDO dash speedo. I have had some that are pretty accurate and a couple that have read quite high.

piperporsche180 · piperporsche180's Garage

I was wondering how 11,000 miles registered on my 951's odometer last summer. Haha. Darn 245/45/16's. They added an extra 440 miles if the error is equal to 7%.

I bought a new set of Pirelli's to put on my spare set of Phone Dials for non track use, 225/50/16 for the rear,205/55/16 up front. Ill retire the 8" and 9" Etoiles and Fuzion ZRI's for track use.

wild man · 03-23-2010, 10:31 AM

Can anyone say if "upgrading" (or is it downgrading?) to 245/50-16's on the back would be enough to eliminate it? I'm going to need to get new rears soon.

P.S. Anyone who is using their 951 for work and getting paid by the mile may want to pretend that they didn't see this thread!

Lawrence Coppari · 03-23-2010, 10:53 AM

Here is how I determined what size to put on the back of my car to get the speedo/odo correct.

I have two cars that read the same - tried them with my wife driving one and I the other. Then compared it to a police speed display on the side of road just to be sure. Then I compared my 951 with 225/50-16 tires on rear to one of other cars and noticed it read about 7% too high.

The diameter of a 225/50-16 tire is: 225*.50*2/25.4+16=24.86 inches.
My speed was about 7% too high so I needed a diameter larger by 7%.

Diameter of a 225/60-16 tire is: 225*.60*2/25.4+16=26.63 inches. This is about 7% larger so it turns 7% slower.

If you have 17 inch wheels, just insert it instead of the 16 I used.

wild man · 03-23-2010, 11:54 AM

Looks like 951S goes 245, instead of 225 (for non-S), due to rear wheels being 1" wider. But let me try the math (need to open up the calculator app). Looks like 24.68" for the 45's, and 25.65" for the 50's. That's only 4% - damn! Looks like I would need to go 255/50-16 to get to 6% (that's close enough for me). Is that size OK for a 16x9" wheel (@ 1" over, I'm thinking it is)? Since the fronts are 50 series, I won't go 60 series on the rears (unless changing fronts also). Probably get better fuelage on 60's though.