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  • Best Rear End Gear for Gas Mileage/Performance.

    Hi all, have been lurking here for a while, but searched and can't find what I was looking for.

    I have a 2001 GMC Sierra with the 4.8L and 4l60e (4wd), RCSB. I believe it has a 3.08 rear end gear in it currently, not sure if it was ordered like that or just fitted from the previous owner. The RPMs are at about 1875 going 60mph and the truck is a real dog off the line (no other issues, fully tuned up, runs and idles like a charm).

    The other issue is that it's real slow when it comes to just driving around on regular streets... assuming it's still related to the rear end. It shifts a lot between 4th and 3rd because I am trying to keep the engine in a decent powerband.

    At any rate, I am wondering what the optimum rear end gear ratio would be for fuel mileage and general performance (just a daily driver, no mods and not planning on it). I think the low gear ratio is affecting gas mileage a bit because it's always shifting back and forth while driving.

    I have had several other NBS GM trucks (a 2004 GMC Sierra 4.8L XCSB and 2005 GMC Sierra 2500HD, XCSB) with seemingly better gear ratios, but I never took the time to decode them, so figured I would check to see what advice anyone here has.

    Thanks!

  • #2
    Re: Best Rear End Gear for Gas Mileage/Performance.

    3.73 would be a good gear. Mine has 3.42 and I also need to re-gear.
    2006 Z-71 5.3
    6" Pro Comp
    35X12.50X20 Cooper STT'S 20x10 XD Rockstars
    Wheatley Tune
    Dual Exhaust Thrush Mufflers
    K&N Filter

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    • #3
      Re: Best Rear End Gear for Gas Mileage/Performance.

      What size tires?

      3.73 would be a good match for you. When I installed 4:10's (when mine was a 4.3L) I gained MPG, the engine wasn't working as hard to get rolling and stay rolling. And it bumped the RPM more into it's power band.

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      • #4
        Re: Best Rear End Gear for Gas Mileage/Performance.

        It has the stock size tires, I believe they are 245/75/16; they certainly aren't low profile sneakers or monster meats.
        Last edited by rc51partsdotcom; 03-01-2010, 12:07 PM.

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        • #5
          Re: Best Rear End Gear for Gas Mileage/Performance.

          my 90 has the 3.73 with a different size tire than what was stock it did have the 275/65/15 but i guess the 2nd owner and me had a hard time finding that size so he went to a 265/70/15 and around town with that gearing i have to be careful on take offs cause to much trottle and the tires break loose lol. but i would go for a 3.73 for the gearing and while you are getting it done get a true locking diff stock one sucks arse.cruising down the highway at 60 it stays around 2 grand which is right there in the power band for optimum mpg

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          • #6
            Re: Best Rear End Gear for Gas Mileage/Performance.

            My dad's 2000 K1500 came with 265/75s and 3.73s and it seems to work good, but it has a 5.3L. So with your 4.8L it should be good.
            sigpic
            1994 GMC Sierra SL
            V6 performance
            http://www.fullsizechevy.com/forum/g...nce-parts.html

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            • #7
              Re: Best Rear End Gear for Gas Mileage/Performance.

              use this chart, helps alot
              http://www.4lo.com/calc/geartable.htm
              with your tires (30") factory 3.42 gear would give you good fuel economy, 3.73's would be good too, and give you better power for pulling if you ever need it
              Last edited by dalaigh; 03-01-2010, 04:22 PM.
              2014 Silverado LT - 5.3, 4x4, Double cab, no mods as of yet…

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              • #8
                Re: Best Rear End Gear for Gas Mileage/Performance.

                Your RPM is 1875 @ 60? You must be geared pretty deep. At 60MPH I'm at or below 1500 RPM and I have 3.73s on 31" tires on my '05 Crew w/ 5.3 and 4L60E.

                - 1993 C1500 - 5.3L/4L60E Swap - 3/5 Hotchkis - '96 Interior - Tuned by EPS -
                - 2013 Camaro 1SS/1LE - Borla - 1" Eibach Drop - Roto Fab Intake - Katzkins Interior -

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                • #9
                  Re: Best Rear End Gear for Gas Mileage/Performance.

                  I think he meant 70, had to because i'm at 2100 at 70 and I have a 5-speed with 3.42 gearing.
                  :fsg:
                  95 Sierra C1500 SL (Sold to father-in-law)
                  99 Suburban K1500 LT

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                  • #10
                    Re: Best Rear End Gear for Gas Mileage/Performance.

                    Nope, he meant 60 (sixty). That's why I am posting... it's such a pain!

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                    • #11
                      Re: Best Rear End Gear for Gas Mileage/Performance.

                      I don't know your tire size, and if I'm doing this right, with 3.08s and 30s your RPMs would be around 1450 at 60 mph in overdrive.
                      sigpic
                      1994 GMC Sierra SL
                      V6 performance
                      http://www.fullsizechevy.com/forum/g...nce-parts.html

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                      • #12
                        Re: Best Rear End Gear for Gas Mileage/Performance.

                        Yes i said 60MPH. If that doesnt sound right maybe it could be wrong but the truck came with 3.73s weather or not that was changed at any time i dont know and keep in mind it does have an after market cheapy tach and at 60MPH it shows 2k

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                        • #13
                          Re: Best Rear End Gear for Gas Mileage/Performance.

                          Again if I'm doing this right, and this isn't taking into consideration any converter slippage, 3.73s, 30s, at 60 mph would put you around 1750 RPMs in overdrive.
                          sigpic
                          1994 GMC Sierra SL
                          V6 performance
                          http://www.fullsizechevy.com/forum/g...nce-parts.html

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                          • #14
                            Re: Best Rear End Gear for Gas Mileage/Performance.

                            Originally posted by 94_c/1500 View Post
                            Again if I'm doing this right, and this isn't taking into consideration any converter slippage, 3.73s, 30s, at 60 mph would put you around 1750 RPMs in overdrive.
                            Your forgetting about the converter lockup. Calculators don't account for that. If he really his turning the RPM he specified then he should have atleast 3.73s already at that rate.
                            Last edited by SimpleGreen; 03-01-2010, 06:52 PM.

                            - 1993 C1500 - 5.3L/4L60E Swap - 3/5 Hotchkis - '96 Interior - Tuned by EPS -
                            - 2013 Camaro 1SS/1LE - Borla - 1" Eibach Drop - Roto Fab Intake - Katzkins Interior -

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                            • #15
                              Re: Best Rear End Gear for Gas Mileage/Performance.

                              I'd do 4.10s. I test drove two trucks just alike, mine with 5.3 auto 3.73s and another with 4.8 auto 4.10s. Honestly the 4.8 with 4.10s felt stronger. But it was a WT model. the one i bought had less miles and was loaded.
                              2004 GMC Seirra ECSB 4X4 5.3 AUTO
                              :fsc:
                              Originally posted by someotherguy
                              It's always easier to ask forgiveness, than ask permission!

                              Richard

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