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Decent cruising RPM

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by 1Blazin71, Jul 16, 2001.

  1. 1Blazin71

    1Blazin71 1/2 ton status

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    Sitting here trying to figure out gears. Im on the fence between 4.88 and 4.56. I have a 700R with lockup and according to my math in 4th gear (.7:1)
    4.88 would put me at 76 MPH at 2500 RPM
    4.56 would put me at 81 MPH at 2500 RPM

    I dont know what the ratio is when I engage the lockup in the converter?

    I get about 11 MPG now with the ZZ4 pretty heavily retarded (38 deg total) to compensate for the too high 3.73s I have now.

    In your opinion, is 2500 too high a cruise RPM?

    Matthew Meyer
    matthewm@inhollywood.com
    '71 Ochre K5 - 4" Lift 35"s
    ZZ4 350 - 700R
     
  2. gokartergo

    gokartergo 3/4 ton status Staff Member Moderator

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    I believe the power band for a 350 is 1800 to 2400 rpm's.....Dan
     
  3. 502Burban

    502Burban 1/2 ton status

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    It seems a little high to me. Your 700R has a pretty low first gear ratio, so 4.11 or 4.56 would probably be best, IMHO. You would still have some good low-end torque with the lower 1st gear.

    Robb
    '73 GMC Suburban - 502DFI
    <a target="_blank" href=http://community.webshots.com/user/robbrj>community.webshots.com/user/robbrj</a>
     
  4. fr8train

    fr8train 1/2 ton status

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    I always thought that 2100-2200 rpm was the best cruising rpm. You didnt specify what tire size you were running I am supposing 35", like your stats say.
     
  5. Thunder

    Thunder 3/4 ton status

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    I would go with the 4.88 or larger ratio.
    700r4s need all the help they can get when pushing a lifted truck and large tires.
    In overdrive,You should be running no less than 2000 rpm at 65 mph.
    Any less than that then the tranny is working too hard without enough fluid pressure and lube. You may burn up the tranny.
    Also. Put on the biggest trans cooler you can get.

    <font color=orange>GOT HORSEPOWER?</font color=orange><P ID="edit"><FONT class="small">Edited by Thunder on 07/17/01 06:26 AM.</FONT></P>
     
  6. 1Blazin71

    1Blazin71 1/2 ton status

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    I've been leaning toward 4.88 with this same thinking, but wasnt sure.

    I do have a lockup converter and a switch to manually engage it. I know 4th is .7:1 but I dont know what the effective ratio is when the converter is locked?

    Anyone?

    Matthew Meyer
    matthewm@inhollywood.com
    '71 Ochre K5 - 4" Lift 35"s
    ZZ4 350 - 700R
     
  7. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    The ratio does not change when the convertor is locked up, its still .7 to 1. Locking up the convertor just removes the 'slip'...

    Rene

    <font color=green>Dyslexics of the world...UNTIE!</font color=green>
     
  8. Stephen

    Stephen 1/2 ton status Moderator Vendor

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    How fast do you typically drive? If you don't get on interstates much, you need to keep that in mind.
    Personally, I'd lean toward the 4.88's. These trucks become real bricks when you go fast, and you want all the power you can get to cruise at high speed. Also, you may end up with taller tires at some point, and the 4.88's would work with 36's, and will help if you have tires with more drag, like a bias ply.

    Ring and pinion strength may be an issue to look at also, but 4.88 is pretty safe in most diffs for most use.

    Making the world better, one truck at a time.
    SW-ORD
     
  9. fatdog

    fatdog Registered Member

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    You can probably figure about a 200 rpm drop when the converter locks up.

    Rob

    Self and Spouse Proclaimed GM Junkie
    She thinks I'm Sick and... She's Right
     
  10. 1Blazin71

    1Blazin71 1/2 ton status

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    Here in So Cal, typical freeway speed is 70-85+. But its only a 8 mile per day daily driver. On the 200-400 mile weekend trips, we usually like to drive between 70 - 80.

    4.88 would put me at about 2300 at 70 MPH, if the lockup shaves 200 RPM, that would be 2100 still at 70 MPH?

    4.56 would be about 2100 at 70 MPH, or 1900 with lockup, which sounds too low RPM to me.

    I think I'm about convinced on the 4.88s. Either way, the gears are going into a 14 FF upgrade

    Matthew Meyer
    matthewm@inhollywood.com
    '71 Ochre K5 - 4" Lift 35"s
    ZZ4 350 - 700R
     
  11. Leadfoot

    Leadfoot 1/2 ton status Premium Member

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    The figures you get with rpm calculators assume converter lock (or no slip), so the 2300 RPM figure is with it locked so if it were unlocked you would raise ~200 RPMs to 2500.

    If you didn't build it yourself, how can you call it yours.......?
     

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