Speedometer in the '07
 

It would appear that the speedometer in the '07 GT is showing 71 mph when actually we are going 65. Without changing the program in the computer or anything else like this, how can it be corrected?

tire size. you would be suprised how much a 1/4 of an inch of diameter distance will make at highway speed.

The car has 18" rims and this information was given when the car was reprogramed for the Ford Racing Cold Air Kit.

Did you give them tire size info too?

What is the tire size on the rear?

Yes to the first question. We have 275-35-18 all the way around. The car came with 235 - 45 - 17 and we switched and changed tires and wheels when the car was new. The speedo has never shown the correct mph.

:hmm:
Seems as though the car came with the wrong tire size initially. It should have came with a 235/55(according to all mustang stuff I have)


Let's do some figuring.

Based off the 18" wheels and the 235/50/18 tires that came on my car, the overall diameter to keep the speedo correct regardless whether you are running a 17", 18", or 20" wheel is a 27.25". 235/55/17's are the same height as well

Your current combination is 25.57" tall which is a good bit shorter overall height than it should be. Now unless there are limits in the FRPP tunes, this should have easily been corrected but I am not positive that the FRPP tunes can adjust the speedo for tire size (I know the Predators and SCT tuners can though)

Since you obviously wanted a wider tire (275's) you could have gotten a closer with a 275/40/18. Its still a little short (26.66") but is closer than a 275/45 (27.75")

I am currently looking at doing a 18x9/18x10 combo for my car and will probably run a 255/45 front and a 285/40 rear. This will keep the overall diameter right at 27" and hopefully the tuner I use will take care of the minute difference.

Does any of that jibberish make sense?

If you would like to see how I figure up the tire sizes above, check out this thread in the Susp, brakes, and tires room

http://www.streetstangs.net/showthread.php?t=12237

It's strange for sure. I would have thought the FRPP tune would have taken into account the tire size. Because obviously FR500's are not stock wheels.

But it's not just his car. Me, in my Cobra...Travis in his 93 Cobra.....Brian in his Whippled 07 Shelby GT with 20" Razors......and Ray in his 07 were all traveling to the Rocket City show in Huntsville. Ray called me and said we were running 80mph when I hadn't run faster than 75. I called Brian in his Shelby and he said we were running 80 also. But when I called Travis, he said we were sitting dead on 75. Mine and Travis' car are still rolling on stock size wheels and tires.

For whatever reason, the tune in Ray's AND Brian's, both S197's with larger wheels/tires are reading faster than they should be. Which, in theory doesn't make sense. Usually adding a bigger tire/wheel combo turns the axle slower and in turn reads lower, not higher. Must be something with the way the S197's new style set-up reads. :shrug:

I am curious on what the guy with Razors is running for tire size.

That's just it. Ray is not running a larger tire combo. He is running a wider tire than stock but it is shorter than stock by over an inch which would make the speedo read faster than actual travel speed.