After further testing (if anyone still cares...)I foundthat in order the make the speedometer match the actual speed, I had to enter 26.5" into the SC tuner as the new tire diameter. The speed correction was done using a Garmin 760 GPS and driving at arange of speeds from 40mph to 80mph, and comparing the speedometer to the GPS. This was done on flat roadways over the period of about two hours.

Anyone have an idea why 26.5" instead of the tire manufacturer computed 27.25" ? Super Chips programmers please feel free to chime in....

 

Thought you might find this interesting

http://www.corvetteactioncenter.com/...s/rearend.html
http://www.ajdesigner.com/phpgear/ge...ire_radius.php

I'd love to drive this car with a set of 4:10s warm up in 1st
and drop it in 2nd

 

I've seen it mentioned that it has to do with the tire being loaded vs. unloaded. Basically the sag in the sidewall caused by the vehicle's weight means the effective diameter is smaller than the raw tire dimensions.

Now that you've pointed it out I'll need to borrow a GPS to see how far off my speedo is now.

 

Hey Redbeard,

In a straight comparison of ride qualities between the stock Michelin tire and the Pilot Sport, I can say:

The OEM tires are quieter, but there is not a great difference.The OEM tiresalso have a better "on center" feel.

The wet traction on both tires is superior, the OEM tires deteriorated rapidly as the tread wore down, at 28,000 the wear bars were almost showing, but the wet performance had dropped significantly.

I am more of road racer, than a quarter miler, so the Pilots do provide a tremendous amount of traction in the curves, but the the OEM tires were some of the best I have ever seen.

The straight line acceleration with the smaller tires is truly amazing. But they seem to have equal traction from a standstill, relatively poor, but that is due more to the excessive weight of the car than the tires.

The Pilots were absolutely worth the money I paid!

The effect of increased acceleration is apparent at all speeds. This is quite possibly the best "sleeper mod" yet discovered.

I would put my totally stock car up against someone with a CAI, long tube headers, and a cat back system, and feel confident that I could take 'em... Boasting? maybe... or maybe not!