Quote:
Originally Posted by sixtyeightC10
I can understand that you're doing this in memory of your father. Both of my parents have passed on so I can relate.
An overdrive trans will help to increase fuel mileage, but not as much as you would hope. This is what can be expected on drive ratio changes; *Note, this statement below is for an automatic trans only. Manual transmissions may vary (but not by much), but will follow the same principal.
The 700R4, 4L60, and the early and late 4L60E overdrive ratio is .70:1. Your current Axle ratio is 4.11:1. The over drive will make the truck feel like that it has a 2.88:1 final drive ratio (4.11 X .70 = 2.877).
So say that you're getting approximately 10 mpg. With a changed final drive ratio you "could" get up to 14.28 mpg. (10 mpg divided by .70 = 14.2857). Of course all of this is theoretical and can't be expected in the real world, especially with the variables of start-up/warm-up, stop-and-go, hills, driving habits, and so forth.
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This is correct as far as it goes, but it's missing a couple of the advantages of the 700R4. First, it has a lower first gear, so the launch is easier, which can also improve fuel mileage. Second, it has a lockup torque convertor, which eliminates slippage at highway speed.