Rear Gears vs Fuel Mileage

Discussion in 'Street Chat / Timeslips' started by RDF1, Jan 12, 2019 at 9:50 PM.

  1. RDF1

    RDF1 I'm Awesome

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    So I've been thinking alot and trying to figure out if this will work or not.

    Quick down and dirty on the setup

    91 C1500
    2wd Regular cab Stepside (4100#)
    5.3 LM7
    Factory 706 heads
    90# bosch injectors
    ISKY 212/212 .561/.561 114 cam
    S475/83 Turbo (W/A intercooled)
    4L80E (2800 circle D stall)
    3.73s
    275/60/17s (30" tire)

    93 octane

    70mph @ 2250 rpms ~18% throttle
    High 15s AFR and 40* timing

    60mph @ 1950 rpms ~13% throttle
    Low 16s AFR and 44* timing



    Last few tanks it's been getting 19-20mpg.

    I would really like to get it up closer to 25 if possible but I feel like with the 3.73s the engine is just turning too many rpms and it's got plenty of power to pull the hills in 4th with the TC locked so maybe less rpms = less fuel if it doesn't need much more throttle.

    I can get another 8.5 rear with 3.08s for $200 and thinking about trying it. Should bring rpms down a good bit. If it doesn't, my buddy can use it for his project truck.

    3.73
    70mph - 2250 rpms

    3.08
    70mph - ~1800-1850 rpms

    I feel like ~400 rpm drop would help Atleast 2-3mpg. Or I'm hoping.

    Ideas?
    Past experiences?
    Suggestions?


    I'm not worried about throttle response, off idle torque, bottom end grunt, low 60' at the track nothing like that. I put this truck together to ride in. And fixin to build a house but it's 48 Miles from work. Which is 24 miles further than what I'm driving now.
    I don't drive any in a city it's all backroads, interstate and highway.
    If the 3.08s work out I'll have to swap them into this rear that I have. It's already got a Eaton LSD and the tubes are welded to the housing and has Moser? Axles.

    Good thing is I could trap in 2nd thru the 1/4 mile with 3.08s hahah.
    7000rpm 2-3 up shift would be ~135mph.









    Sent from my HiDiNg PlAcE
     
  2. PlayingWithTBI

    PlayingWithTBI Newbie

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    With that cam and TC locked @ 70MPH I would think you'd be too far out of your power curve to pull hills comfortably (3.23 gears would get you closer to 2,000RPM @ 70MPH). If it's only gonna cost you some labor to find out, might as well try it...
     
  3. Supercharged111

    Supercharged111 I'm Awesome

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    Do a T56 swap. Best of every world.
     
  4. RDF1

    RDF1 I'm Awesome

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    It chugs along good down to 1200rpms.
    I had 3.42 in my other turbo truck that was heavier and it was [email protected] It towed like a champ so that's why I was thinking going even higher gears in this lighter truck.

    Sent from my HiDiNg PlAcE
     

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