Yeah, going from 3.73 to 3.42 is not gonna get you much at all...and it would take a long time to pay for the parts and install, plus reflash of the computer, etc...is your truck 4x4? If so, you'd need to do all of this in the front at the same time as well. You'd want to use a quality ring and pinion set, cause if you go with a junk import cheapie, you'll likely experience gear whine and who wants that in a nice quiet Tahoe/Yukon? R&P set alone will cost you probably $300-400 for a quality set, plus the master install kit (bearings, shims, etc)...unless you know how to swap a ring and pinion, you're looking at several hours of labor charges. Then when the gears are done, you'll need to have the computer reprogrammed at a dealer or someone with the equipment.
If you're getting 17mpg combined, I'd be pretty happy with that...my average is 15mpg pretty constantly, unless I go on a long highway trip, which is usually about 17-18mpg if I drive smoothly...and I have the 3.73s.
Some quick math - if you somehow picked up 2mpg (which I doubt), that would get you basically 50 extra miles per tank (25 gallon tank). Let's just say 19mpg (in your hypothetical case) now. If you drive 12000 miles a year, at the 17mpg, using $4.00/gal, you'd pay $2823 in gas. At 19mpg, you'd pay $2526. So, you might save $300 a year if you were somehow able to increase your mpg by 2mpg with the gear swap, and I really doubt you will get anywhere near that going from 3.73 to 3.42. In fact, there is a chance that your mileage might actually go down, as many have shown that the 4.10 ratio might actually help mpg because of it's torque multiplication benefits in getting the vehicle moving.
In my view, you could spend your money much more wisely than going with this swap...it will take you several years to pay off the cost, and doubly so if you have 4wd!
Oh, and swapping in a new 6 speed auto would be WAY more expensive! I'm sure you could get someone to do it for you, if you can get your hands on one in the first place...but you're likely looking at big $$s for a swap like this, plus losing your warranty for the drivetrain...and I'm sure you'd need a new computer to get the engine/new tranny all talking together and working right...big headache I'm sure!
Last edited by kpanza; 06-13-2008 at 02:14 PM.