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Discussion Starter #1
I want to get an LSD for my 3.9 automatic and wondering if 4.10s will decrease MPG. I have the stock 235/75r15 and want to go 18s or 20s with 255s later. Should I get 4.10s or would 3.92s be a better choice? I get about 20 MPG on the highway and not sure about city. I have 3.55s right now.
 

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Home School Valedictorian
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Likely loose mpg's, usually is a slight mpg increase in town but a decrease on the highway. There are online tire/wheel/gear calculators I would determine what tire size you want to go with, then compare the effective gear ratios of the current and future set-up, mess with the calculators and see what the different gear ratios do to the rpm at specific mph.
www.tirerack.com has great specs on all of the tires they sell, you can get all of the specific info needed for the calculators.
 

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Home School Valedictorian
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I didn't lose anything going from 3.92 to 4.10. If anything my MPG got better.
Didn't you comment recently that there was a tire size change also?
All things being equal, you do not increase highway rpm's without a drop in mpg's.
 

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Didn't you comment recently that there was a tire size change also?
All things being equal, you do not increase highway rpm's without a drop in mpg's.
I've been running 4.10s for 5 years now. Just recently switched back from 255/50 on the rear to the stock 255/55. With my motor set up, it runs better at higher RPMs. Using the overhead computer, from about 65mph through 80mph there is barely a 1mpg difference in gas mileage with the cruise set.
 

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Discussion Starter #6
Ok thanks. Where would be a good place to get gears and what brand?
 

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SIXER34
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Any kind of Motive Gear from Advance Auto is a good one. I have 4:10s with LSD and I get 22mpg at 55mph. And thats with a 4.7 v8 4x4 Quad Cab!
The increase in rpms is true, but the truck doesn't have to work NEAR as hard to move our HEAVY mid size trucks. And the increase in performance is real nice too.
 
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