Quote:
Originally Posted by kglowacky
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.
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Quote:
Originally Posted by GRADYS Performance
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.
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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.