David Porter

I have a 79 F-100 with a 9 inch rear end, C-6 trans. I believe it has the stock rear end gears, because it really runs in a high RPM band at about 60mph. Any suggestions on gear sizes for more speed, less torque? I'm building it as a street truck more than anything. Thanks for any suggestions.

David Porter

Nobody?

JayzDaddy

What engine? 302, you dont want to go to high because it will fall over itself taking off. 351, you have more wiggle room due to more torque.

A 3.05 may be a decent median.

Google gear ratio calculator.
You can plug in your tires, tranny final drive, speed, and rpm at that speed and it will kick out a number for a ratio.

Matthew Burr

may be simpler to get taller tires on it too :P

jkenprice1

What you need to find out is what gear ratio you have now. One way to do that is to jack up a rear tire. Put a mark on the drive shaft and on the tire. Turn the tire 1 complete revolution, counting the number of revolutions the driveshaft make. If you have a 4.10 ratio, the driveshaft will make just over 4 revolutions. If it's a 3.30, about 3.3 revolutions, Having a tach helps also. Most trucks are running between 1300 and 2200 rpm's at 60 mph. Around 1800 to 2000 is good for a street truck around town Good accleration, Ok rpm's. For highway driving, lower rpm's are better. I'm not sure what the ratio's were in 1979, yoou could probably find that info with a little research. For the newer trucks, I think they are 3.10, 3.30, 3.55, 3.73, 4.10. I've read that for every ratio change, the engine speed changes about 200 rpm. Hope this helps

David Porter

Thanks for the info so far. It is indeed a 351C, C-6 A/T, Factory A/C, 650CFM 4BBL, Mild Cam, Headers, etc... I am going to check the drive-shaft rotations tomorrow. I guess I need to find a decent Tach for it as well. I did go from a 75 series tire to a 60 series. I didnt like the taller tires on a 2wd.