33s, Gears, and MPG

Discussion in 'Lifted Supension - Z71 & 4x4' started by WildBill, Jul 12, 2018.

  1. WildBill

    WildBill Full Access Member

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    So I've done some searching on this but I don't have a clear answer...

    I recently added 305/55/20s to my 09 suburban and I find that it likes to drop to 5th when going up even small hills. This is hard on MPGs. I have 3.42 gears.

    Has anyone on here upgraded to 33s and then changed over to 3.73s or 4.10s? Did the steeper gears improve mileage any?
     
  2. swathdiver

    swathdiver Full Access Member

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    Let's play with some numbers. Your tire size mathematically works out to 33.21 inches tall. Stock tire (265-70-17) is 31.61 inches.

    Stock tires, 70 mph, 5th gear is about 2,175 rpms and in 6th 1,700 rpms.
    33s, 70 mph, 5th gear is about 2,075 rpms and in 6th, 1625 rpms.

    Now put a 3.73 ring and pinion set with those 33s and she'll turn about 2,250 rpms in 5th and 1,775 rpms in 6th at 70 mph.

    With a 4.10 gear and the 33s she'll turn about 2,475 and 1,950 rpms respectively.
     
  3. WildBill

    WildBill Full Access Member

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    Yep that looks about right.

    Has anyone swapped gears after a tire change? was there any improvement in milage?

    (I know it's higher rpm, but it's also less downshifting to 5th and less load on the engine)
     
  4. Matt_

    Matt_ Full Access Member

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    I did, it definitely helps but don't expect stock gas mileage. My city driving gas mileage improved more than highway, but my highway mileage can tank pretty quick if I want to cruise at 80. It drinks A LOT less in the 2000-2200 rpm range than 2400+

    Mines 6.0 with 6L90E, stock gears were 3.73, new gears are 4.56, running 35s... YMMV (literally)
     
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  5. WildBill

    WildBill Full Access Member

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    Cool.

    Anybody done just a modest change to say a 3.73?
     
  6. TxDrewski

    TxDrewski Full Access Member

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    IMHO, the money invested to do the swap is not worth the slight MPG increase your going to net. The cost (in my exp), would be better used on a tune, or just to put back in the fuel tank lol. i should note my gear swaps were all on 4x4s, so I had to change gears in the front and rear, so my cost was double what yours would be.
     
  7. WildBill

    WildBill Full Access Member

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    I'm 4x4 too. What was your cost to re-gear? What ratio? How did it affect your mileage?
     
  8. Flexx

    Flexx Full Access Member

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    What is your actual mpg?

    Just switched from NBS tahoe on 35"s to an 07 suburban, all stock now, ordered a 2.5"/1.5" kit. Trying to decide what tires to go with. Like the look of 305/55/20 but could do 275/65/20 or 285/55/20

    Sent from my SAMSUNG-SM-G891A using Tapatalk
     
  9. jlee

    jlee TYF Newbie

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    I just went a 2in rough country up front and kept my autoride on my 09 Tahoe. I'm just a little higher in the rear yet when the shocks are pumped. I'm running stock 20's with BFG 285/55r20 which I think are perfect, no trim required but a noticable increase in width and height. I dropped from about 16.9 mpg with my Duelers to 15.7 mpg which I think is still pretty good for how I drive. I'm at 43 psi with the BFG's right now and still get a factory like ride and not much road noise. I have 3.43 gears by the way.
     
    Last edited: Jul 30, 2018
  10. Flexx

    Flexx Full Access Member

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    Wow. If i can get 15 highway I'll be pumped. I'm going to stay stock tires for now until they wear out or i score a deal. Will also keep stock rims. The stock 20" wheels are my favorite factory rims ever made.

    My tahoe on the 35" tires got 13 highway.

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