: Gear Ratios

09-05-2007, 04:25 PM
OK guys, I am dropping this Cummins into my chevy C20. The way I figure it, I will need a 2.73:1 gear ratio to acheive 55 mph at about 1800 rpms. Have any of you had this problem. My gear ratio is 4.10:1 now and I am using the 3 speed automatic trans behind the 90 model cummins. this seems like a high ratio for a truck. Can you help me in this matter?

Curtis Traylor

09-05-2007, 11:03 PM
It all depends on the the size of tire you use. The larger the tire the more your RPM will come down. You must be careful to not run too large of a tire or it will take longer to spool up your turbo due to the engine will be pulling as if it was in the wrong gear. This would produce alot of black smoke and have a ton of turbo lag.

The other side of the coin is if you use to small of a tire your power and boost range will be quick and feel like your in 3rd before you have any speed. 4:10 R&P's are very low for the RPM range of the 4BT but you can help that by increasing your tire size.

Hope I helped, if not :happyfinger: HEHE

09-06-2007, 03:58 AM
The TF727 are a stout box, the 1st gen dodges used a very high geared R&P to compensate, if you have to run it though how about an overdrive?