Correct Tire Diameter?

February 28, 2006, 01:07:43 PM

Hey all,
I've got an buttstuff question for somebody. My '83 Cougar came with the TRX package originally, so it had the funky-sized 220/55R390 tires. That equates to roughly 24.9" diameter. It seems that the norm for '83 to '86 Cougars and Birds is in the neighborhood of 25.6" to 25.8" diameter. Here's the question: were the TRX cars actually corrected for the diameter difference (because of the size limitations of the TRX tires) on the speedo gearing, or was the 25.7"-diameter speedo gearing used, knowing that it would a certain percentage off? Yeah, yeah, I know, who cares? Here's reason #1 I'm asking. I'm replacing the TRX wheels with 16" snowflakes. I have the opportunity to pick up a like-new set of Falken 225/50R16's fairly reasonable, which also yield a 24.9" diameter, same as the TRX tires. However, I already have a pair of 245/50R16's (50% tread left) mounted that could be used for a while longer, and they are 25.65" diameter. Which should I put on? Reason #2 for my question is, I'm also about to mount an '88 TC 8.8 rear under the car with 3.55 gearing, which will replace the stock 3.08. So I know I have to correct for the gearing change anyway. So if I keep the bigger rear tires, do I need to factor that in to the 3.08-to-3.55 change? Can you tell I'm an engineer by trade? Like I've said before, I just like knowing that I'm seeing the right speed and showing the right number of miles traveled. My car has the digital speedo, too, if that makes any difference. Thanks for the feedback.

Fordman3

Correct Tire Diameter?

Reply #1 – February 28, 2006, 04:38:41 PM

Ha, I was going to come up in here spouting off those same numbers, but it looks like you already know. If I were you, I'd use the 25.7" OD with the 3.55's. IMO 25.7 is still too short for a car this size (which is why I switched to 235/60/15's, 26.1' di, and doesn't throw off the speedo for a 25.7di size) . As I'm sure you know, a smaller diameter tire (shorter circumference) will not last as many miles as one with a larger circumference due to the fact that smaller tire will turn more overall complete revolutions at the same speed as the larger. So the tread surface of the smaller tire with techincally cover more miles than that of a larger diameter tire. With the weight of these cars factored in to that also, even more reason to use the larger size (besides having a quieter ride and better mpg etc.) Most speedo gears and tire sizes for fox cars in general are scaled towards the 25.7' di since that's also the standard size for the mustang GT.