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3:89 Re w 26" tall tires, but they are too narrow. So I want to put on 275 60 15's which are 28' tall. Clearence on the car is not an issue there is plenty. given the added dia. I am sure rpm will decrease some. Which will not break my heart. Those 3:89's make the engine howl pretty good at 70 MPH (459 dyno'd w drivetrain load HP). But how much will the added 2" change it?
the difference will be proportional to the change in tire diameter: 3.89:1 with 28" tire = 3.61:1 with 26" tire.... at the same MPH, the RPM will drop 13/14ths (26/28ths).
-Chris
Skids typed quicker than me :LOL: With your 3.89's and no OD (1:1 final tranny output) you'll turn 3519rpm at 70mph with 26" tires, and 3268rpm with 28" tires which is indeed exactly 13/14th's change.
Here's a great little calculator to figure it out..http://www.ringpinion.com/Calc_RPM.aspx
Plug in all the different combos you want and see how it changes your rpms....