Changing rear diff gear ratio.

Big Display · December 5, 2006

Currently I have a 69 gmc C1500 with a 454 in it with 4.11:1 gear ratio in the rear diff and a m21 tranny. The truck has really good acceleration, but it get wrapped up pretty fast. I'm looking to swap the rear diff gears to either a 3.01:1 or a 3.42:1 to make a little better on fuel. Does anyone know where I'll loose the most power? And which gear ratio would be better to go with. Any suggestions or comments would be greatly appreciated!!

svensk · December 5, 2006

It doesn't make sense to spend all that money swapping gears just to save a few bucks on gas. You know how long it would take you to recoup a grand or two in gas savings? If you have a nice old truck like I would do the opposite and turn it into a hot rod.

In the end if you did the gear swap you would be down alot of money, would have a truck that accelerates like a glacier and still only gets 7mpg.

Clavin78 · December 6, 2006

Yeah that 3.01 would dog it everywhere

95Sierra2500 · December 6, 2006

If you wanted to go anywhere I'd recommend 3.73's. You'd still have plenty of power available (3.42's or 3.01's would kill your performance) and improve your mileage, but the most you're really going to gain is probably 1 or 2 mpg. It IS a big block with (presumably) a carb on it, and those only got like 7 mpg anyway.

Big Display · December 6, 2006

Thanks for the info. It makes sense that going to all the trouble of changing the gears just to save a little on gas isnt a real good idea. What about getting a 5 or 6 speed transmission? I currently have a 4-speed. But it seems to me that a new transmission could give me more speed while lowering the rpm's. Once again that would be more money to save a little on gas, but it at least wouldn't have the wear on my engine right? If you can't tell, I'm just getting into the performance world and all the help is really appreciated.