How much deviation...
 

According to the "Wheel and Tire FAQ" Injuneer posted the following-

TIRE SIZE: TIRE DIAMETER; WHEEL WIDTH

245/50-16: 25.7” diameter; 7.0” – 8.5” wide wheel
255/50-16: 26.0” diameter; 7.0” - 9.0” wide wheel

245/45-17: 25.7” diameter; 7.5” – 9.0” wide wheel
275/40-17: 25.7” diameter; 9.0” – 11.0” wide wheel
285/40-17: 26.1” diameter; 9.5” – 11.0” wide wheel
315/35-17: 25.7” diameter; 10.5” – 13.0” wide wheel

Now, I want to run this wheel: http://www.konigwheels.com/Konig-Hom...els/BUMP-BLACK

Preferably in a 17X7 diameter but all of the tires that I'm looking at on discount tire don't have tires in the "standard" size. Based on the calculations I have using the FAQ, a 255/45/17 tire comes out to a total diameter of 661.3mm, higher than the 651.1mm range of the stock tires. And 225/50/17 comes out to 631.4mm.

It looks like the 16X7 wheel will not work either because the 235/55/16 tire has a diameter of 664.9mm and the 255/50/16 has a diameter of 661.4mm

So my question is how far can the diameter deviate from the stock number before the speedometer must be calibrated?

Just seems like this wheel is not going to work out no matter what. Is there some way that this can fit on a 16 or 17" wheel?

Your speedometer will be off by the same percentage that the tire you choose is different from the stock size- put a tire 10% larger or smaller, and your speedo will be off the same amount- If the difference is too great, it affects not only the speedo, but the shift points if you have an automatic transmission. Easily programmed out, either with an aftermarket tune of one of the hand-held units like a Hypertech or a Predator tuner.
Obviously, in a perfect world, you would try to maintain the same rolling diameter. Pretty close in good, if you get too far away, then a tune would become advisable.

Thank you very much. I basically gave up on that wheel because I want to run different wheels and tires when I go to the track. I have a 1993 T/A, so reprogramming the existing PCM is not possible unfortunately. I am doing a massive overhaul right now and am getting a new PCM ANYWAY, but I'd rather run wide DR's at the track.

Personally, I think going to a 7-inch-wide wheel is a mistake. The factory 16" wheels are 8" wide, and the factory 17" wheels are 9" wide. Technically you can mount a 245mm tire on a 7" wheel, but it's stretching it more than I'd be comfortable with.

Anyway, the Konig Bump Black wheels aren't available in the 5x4.75" bolt pattern necessary to bolt up to your Trans Am.

Yes, I seen that after the fact as well. (the bolt pattern incompatibility) I am having a difficult time finding a wheel that I like in the 17X9" range. I'm not a big fan of the 5 spoke/star=style wheels. I'm still searching all day long for a viable alternative. Thanks very much for the advice on the diameter, the Tire FAQ has been a big help but there's still a lot I don't understand about tires/wheels.

Yeah, unfortunately, our bolt pattern and width requirements narrow your options significantly. The well-known wheel companies like Konig and Enkei target mostly import cars, which don't use the 5x4.75" bolt pattern.

I recommend sticking with one of the following sizes:

245/50R16 on 16x8 wheels
275/40R17 on 17x9 wheels
315/35R17 on 17x11 wheels (plan on some minor modification to keep the fender lip from cutting the tires)
275/35R18 on 18x9 wheels

All of the above are within 0.5% of the stock tire diameter and should therefore require no reprogramming to keep your speedometer and shift points accurate. I have 275/40R17 on 17x9 wheels on my Camaro.

They're pretty expensive, but Forgeline makes excellent wheels (including lots that aren't 5-spokes) that will work for your car.

Thank you again for your help. I placed a bid on some American Racing 17X9's on eBay before I seen your forgeline recommendation. They're here:

http://cgi.ebay.com/ebaymotors/ws/eB...STRK:MEBIDX:IT

If there's any style of star-shaped wheel I like it's this one. Wanted to wrap them in Nitto INVO's, but they only have them in

245/40/17 and 245/40/17.

So I'm going with NT555's, not the DR's though. I want to run 315's and difference wheels when I go to the track.

You don't reprogram the ECM to correct the 93 speedometer. It has a different vehicle speed sensor (VSS) than the 94 and up cars, and the speedo can be calibrated by changing a gear in the VSS, or by putting a "box" like the one from Dakota Digital in the wire from the VSS to the speedo (all the other years send the VSS only to the PCM).

Thank you so much as always, Injuneer. This is particularly helpful information. I will however heed the advice of you and the others that contributed to this thread and the Tire FAQ and stick with conventional wheel diameter/tire setups.