choosing rear end gear ratio

[nitro640]

is there a way to figure out what gear ratio to use? I have a 67 fairlane and i want to put some gears in it but can't decide on 3.80 or 4.11. it will be a street/strip car no daily driver.

[Macgyver55]

To determine the proper gear for your car you need to take into consideration what the motor is built for (torque & HP) and what it is capable of RPM wise. Generally a deeper gear will give you the best hole shot, but a low RPM/high torque big block, combined with a tire that is too short you may run out of rpm before the end of the track if the motor is not capable of running a high enough RPM. Or a high RPM small block may not have the torque at low RPM to get moving quickly with a tall gear and tall tire and have lots of RPM left at the end of the track. If you already have the tires you want to use and know the RPM range where it makes the most power the rest is simple.

The formula to calculate RPM at any given speed is

rpm = (mph x final gear ratio x 336*) divided by the tire diameter

If you want a weekend warrior that is good for both quickness and streetability than a compromise can be figured out with a taller or shorter tire combined with the gear you choose.

[nitro640]

thanks for the reply macgyver, it is a .04 over small block good for about 5200 RPM, right now it has 2.79 gears one wheel wonder, I ran it at BIR this fall, ran 16.xx at 85 mph, had traction problems and it didn't shift into third until the last 100 200 feet. I'm going to do gears and a posi or locker this winter,I'm hoping to run around 100 in the 1/4. next year I will do heads and a bigger cam.