Forum Discussion

76_Rambler's avatar
76_Rambler
Explorer
Nov 01, 2020

Rear gear ratio

I bought a 24 ft class c 1976 Holiday Rambler (E-350 chassis) last year, and recently had the motor rebuilt (ford 460). I ditched the motorcraft 4350 carb, and went with a holley 650 dp. I had them install a bigger cam on rebuild. It absolutely has plenty of power taking off from a stop, or climbing hills with no downshift. The transmission is a c6. The good thing is that it's a c6. The bad thing is that it's a c6, and has no overdrive, which means I'm running 2700 rpm at 55 mph and 3150 at 65. It was built when the federal speed limit was 55, and geared for that. I'd like to put a taller rear end in it to reduce my rpm's at 65. I'm just looking to see if anybody else has been through this scenario, and how far is too far on swapping the ratio. I know I'm going to lose power on the low end, and at the lower end of a shift. I'm sure I can make up for any bogging down after shifting gears with a stiffer shift modulator spring to shift at a higher rpm. I haven't lifted it yet to determine what's in it now.
Class C
  • whemme's avatar
    whemme
    Explorer II
    Nov 02, 2020
    time2roll wrote:
    whemme wrote:
    How was it possible to install a bigger cam. I can’t see how a bigger cam would fit into the existing bearing journals that hold the cam.
    The lobes that activate the valves/lifters are taller and longer duration of lift.

    That is a cam with modified cam lobe profiles - not a bigger cam!
  • time2roll's avatar
    time2roll
    Nomad
    Nov 02, 2020
    whemme wrote:
    How was it possible to install a bigger cam. I can’t see how a bigger cam would fit into the existing bearing journals that hold the cam.
    The lobes that activate the valves/lifters are taller and longer duration of lift.

    For the Op... you probably just want to move about 10% so 4.56 to 4.10 would be an example or 4.10 to 3.73 depending of course on what is available. Your RPM will move down the exact same percentage. Speedometer will need corrected, I think a replacement gear on the speedo cable.
  • whemme's avatar
    whemme
    Explorer II
    Nov 02, 2020
    How was it possible to install a bigger cam. I can’t see how a bigger cam would fit into the existing bearing journals that hold the cam.
  • Grit_dog's avatar
    Grit_dog
    Navigator
    Nov 02, 2020
    ^ But re-gearing it will be waaay cheaper, albeit not as useful as a GV.
    It likely has 4.10s, maybe 4.56 but doubt it. Short tires are contributing to the rpms as well.
    Healthy big block will pull a lot but I wouldn’t go any taller than 3.73s, unless you’re just driving around the prairies. Then you could probably gear it up a bit more.
  • T18skyguy's avatar
    T18skyguy
    Explorer
    Nov 02, 2020
    You might look into a Gear Vendors Overdrive. It works great with the C6 and you'll have 6 speeds. It's a nice setup and they been around forever.
  • Lwiddis's avatar
    Lwiddis
    Explorer II
    Nov 01, 2020
    2700 rpm at 55 mph? Yes, time for a change. What rear end do you have now?

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