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Hi everyone.

Presently I have P225 70 14 tires on my 69 Chevelle. The wheels are the original factory SS wheels. The reared gears are 3:73's. I turn approximately 3200 rpm's at 55 mph or so. The speedometer doesn't read accurately. I am considering putting 3:31 gears in the rear. My question to everyone is, would a taller tire make much of a difference in the RPM output? I would like to make the car a little more cruise friendly. I want to keep the same wheels. I know the gear change would make a difference but I was thinking the taller tire may make all the difference instead. Does anyone know how tall I can go with a new tire?
 

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Hi everyone.

Presently I have P225 70 14 tires on my 69 Chevelle. The wheels are the original factory SS wheels. The reared gears are 3:73's. I turn approximately 3200 rpm's at 55 mph or so. The speedometer doesn't read accurately. I am considering putting 3:31 gears in the rear. My question to everyone is, would a taller tire make much of a difference in the RPM output? I would like to make the car a little more cruise friendly. I want to keep the same wheels. I know the gear change would make a difference but I was thinking the taller tire may make all the difference instead. Does anyone know how tall I can go with a new tire?
Tire sizes for 14" rims can be limited. It's worth looking at tire manufacturers websites to see what is available then find an online gear ratio calculator and crunch some numbers before making any decision.

Steve R
 

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RPM = mph x gear ratio x 336
-------------------------
tire diameter

That said, a 15 inch wheel with something like a 255x70R15 tire would help. an overdrive trans would help, also
 

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3.73 to 3.31 is about a 10% change

A 2" taller tire is about a 7% change
 
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