I was wondering would it be worthed to program the computer for a different size tires? I went from 16 in salad shooters to 17in SS rims. Because when I went to the gear calculator and with 275-40-17 and 3.42s it looked different from what shows currently on the speedo. Also has anyone done this. Any help would be greatly apprecaited. Thanks.

 

Nah, just leave it as is. The 275/40/17 has the same overall height as a 245/50/16.

(275 * 0.40 * 2)/25.4 + 17 = 25.66"
(245 * 0.50 * 2)/25.4 + 16 = 25.64"

PS. Don't worry about the gear calculator either. Those are theoretical values only and it ignores such factors as friction, etc.

 

I don't think so, they should be the same size. And if you really want it reprogrammed go custom tuning pcmforless.com or madz28.com, definitly not HPP.

 

I don't understand. The last time I measured my 245/50/16 tires they came up as 24.75"

Where do you get the 25.63" from. I ran 265/50/16 tires with my 3.73's and they measured an inch taller at 25.75.

I've seen guys posting about their tachs or speedos being off.
Maybe this is where the problem lies.

My speedo is right on and I show 3000 rpm at 80 with the stock tires and 2800 rpm at 80 with the taller tires.

 

madwolf showed the calculation, based on the section width, profile ratio and wheel diameter. You can also check the published dimensions for any tire on Tire Rack. 245/50-16's generally show up as 25.6" or 25.7". My original 245/50-16 GY GS-C's were that diameter. You can also look at the stock PCM programming, and GM uses 25.65" for the 245/50-16's.

How did you measure the diameter? The most accurate way is to put a mark at the bottom of the tire, and a matching mark on the pavement. Roll the car ahead until the mark on the tire is at the bottom again. Put another mark on the pavement. Measure the distance between the two marks on the pavement. Divide this distance by 3.1416 (pi) and that will be your diameter... example 80.5" / 3.1416 = 25.62"

That's an unusual size tire... I think only BFG makes that in their "Radial T/A" line. They quote the diameter as 26.5", and it calculates out to 26.43" using the method shown by madwolf.

 

Well I just use a tape measure with the tire mounted on a rim.Even off the car it still measures 24.75 for a stock 245/50/16. Why don't you measure the actual height with a tape measure instead of using a math formula to figure it out.

The tape measure doesn't lie and why use some formula anyways?

 

tread depth between the tires could have been differnt also

 

Welllll..... even the "math formula" starts with a tape measure.

You need to know the "rolling radius" of the tire. Thats the height under load, and the dimension that affects speedometer accuracy. Rolling radius is different than the measured diameter of the tire. It compensates for the compression that occurs under load. That's why measuring the distance moved forward in one rotation is reliable.

Typical tread depth for a new street tire is 11/32". If your tires were bald, its conceivable that the actual diameter could be 25.65 - 0.69 = 24.96"

Not sure what your point is.... GM says the 245/50-16 tires they put on the F-Body are 25.65". All the tire manufacturers publish that it is 25.6" to 25.7". I guess your saying they are all wrong?????

 

I find it hard to believe a tire will grow an inch while driving and will continue using what my own eyes tell me because my speedo is right on. I suggest you measure your tires and decide if you want to use the 25.6 number instead of 24.75.