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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.
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