Originally posted by fiftee6
OK, so doing it the way typhoonZR said, in one full rotation of the tires (on the ground), the shaft spun 3 and about an inch which would in that theory be approx. 3:10 ratio, however, because both tires are on the ground and the car originally had 14's not 15 inch tires, wouldnt that put the rear gears at about 2:90? That would seem about correct, assuming that ratio was avail then.
So, if all this is true, and I go to 3:73's, this then will be a noticable difference off the line, True? And one more thing. If at 60mph Im tachin about 2400 rpms, what will rpm's go to at 60 with the new rear???

Phil...
Hey 56;

You are close, however wheel and tire size have absolutely no bearing on determining gear ratios in the method given. One revolution of the axles is one revolution of the axles. So even if you have the wheels and drums off, if you turned the axles one rotation, it’s still all the same, even though the diameter of the axles are only about six inches, it just isn’t a factor..

I believe where you are getting confused is in this. If you install larger diameter tires, your effective gear ration will drop. What I mean to say is that the rpm of your motor will be less, given you are driving the exact same speed (not according to your speedometer) on the highway. So say you have 3.08 gears with a 24 inch tall tire, and then you install 26 inch tires, your rpm will drop just as if you had installed a taller gear, such as a set of 2.73. That is just a guess. I am sure, someone on this site might want to bother calculating the exact effective ratio, but all I am doing is giving you an example.

Likewise, say you have 3.08 gears with 24 inch tall tires and then went to a 22 inch tall tire, your effective ration would be steeper, as if you installed a higher numerically gear ration, or as they might say in automotive land, lower gears. I know, go figure.

So, 56, from what you stated, my guess is that your gear ration is 3.08, maybe 3.23. In any case, you now know that your gear ration needs to be changed to a numerically higher number, such as 3.73- 4.10. This will make a HUGE difference is your “seat-of-the-pants-o-meter.”

Your rpms would jump from 2400 rpm to about 3100. Just guessing