rear diff change
 

Going to change the rear diff in my 78 . Have a 370 with a LS1 / 460E in front of it. The gas mileage is poor. Thinking of going to a 3.08. Really don't need the ability to smoke the tires every time I leave a stop sign. Been years since I have been in the market for a rear diff but I would think a 3.08 should be a cheap purchase or a easy trade for my 370. Thoughts.

Re: rear diff change
 

If you’re thinking less rpm equals better mpg, it’s likely not true in your set up.
For best mpg you’re likely turning about 300 rpm lower than optimum. Assuming 28 inch tall tire you’re near 2200 rpm at 70mph. 2500 rpm is likely where your motors sweet spot is.
Going to a 4.10 gear would be better.

Re: rear diff change
 

I think you're on the right track as long as you can get a 308 for cheap. Might take a while to save the two or $300 if you have to buy one, depending on how often you drive the truck, but if you keep it I would imagine you'll get there. I have a 308 open that I'd trade you, but we'd end up spending way too much money on freight. If you can wait a couple summers, I might be able to bring it down to C10s in the park. What do you think about letting us know what you find and how it works out?

Re: rear diff change
 

The 4L60E transmission has a gear ratio of .70 to one in 4th gear. that make you 3.73 a 2.61 in Overdrive. A 3.08 becomes 2.15. Way to low for the RPM your engine need to run in going down the road to be efficient. As a previous poster said 4.10 would probably be better as is is only a 2.87 in overdrive. You need to look at MPH, RPM, Gear Ratio and Tire Diameter to figure out where your engine will run rpm wise and ear it accordingly.

TCIAUTO.com has a tech reference section on their website that has some calculators that will easily help you with the formulas.

Your engine has a rpm range were it will get the best mileage based on how it is built. Cam, Heads, Intake, Carb ETC. Figuring that rpm range out and then gearing it accordingly will get you the best result for good fuel mileage.

Running the engine to low of rpm can hurt mileage as much or more than running it to high.

Re: rear diff change
 

What gas mileage are you getting now? It's probably not going to do better than 16-18 mpg at the best.

You could try running it down the highway in D instead of O and see what happens to your mileage. If it actually goes up then you need a lower gear as suggested (or you could just live with three gears).

One time I drove up to the High Line to meet somebody on the train. Most of the way there I discovered I was still in D not O. Still the round trip was about 16 mpg which seemed pretty good for a 4x4 Suburban running 65-70 mph with the AC on the whole way.

Re: rear diff change
 

The calculator at grimmjeeper.com is second to none. Figure out your target rpm at cruising speed, and plug the numbers into that calculator.

Re: rear diff change
 

If you were in a flat environment, like down here on MS coast, it would probably be OK with a 3.08. Heck GM did it. In reality I agree that a 3.73 or 4.11 would be a better combo for hilly areas, especially if you use the truck.

Re: rear diff change
 

As long as your engine makes sufficient torque to push the vehicle so that it's not shifting in and out of overdrive on the highway, you're in good shape. Your tire size is the missing ingredient and is important. The weight of your vehicle and how much in-town driving you do are also important. You do have to have a combination that will get you into your torque curve for start / stop and reasonable acceleration. Or around town will eat up the highway gains.

Re: rear diff change
 

Yeah, tires are cheaper and easier than changing axle gears.

I had a 1980 K10 with a four speed SM465 but the top gear was basically an overdrive because of the 3.07(?) gear ratios. You had to be going about 35 to get in 4th (3rd per the knob: L123), and you could run 50-60 mph up a hill in 3rd pulling a trailer.

Re: rear diff change
 

My 87 R10 came factory with a 350/700R4 and 2.73 rear end.

Kglowacky, if you were closer, I have a 3.08 geared rear that you could use to see if that combo would work for you. That way you would only be out of some time if it didn’t, instead of money