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Thread: highway speed issues.

  1. #1
    Join Date
    Feb 2009
    Location
    Huntington Beach, CA
    Posts
    22

    Default highway speed issues.

    Just got my m715 in jan. and is going to be my everyday driver, it has a ford 460/c6 combo in it. and i just got back today from a trip out to the desert to do some off roading.

    off road the thing is a wonderful.
    getting to the off road some 180 miles from my house was not so wonderful.
    top speed of around 50-55 at 3000rpms. and gear oil leaking out of the t-case everywhere.

    was curious what the best options are to go about getting much more manageable highway speeds but still have it crawl rocks well.

    -over drive unit bolted to the back of the c6 tran.?
    -no doubt reading around on the fourm i will have to change the t case?? to a np205 will it be able to handle the highway speeds?
    -and should i change the stock axle gears?

    alot to think about already and i just bought it...

    dan

  2. #2
    Join Date
    Sep 1998
    Location
    North Central Wisconsin
    Posts
    11,520

    Default

    The NP205 is a great swap.

    What you do about getting more speed does depend on how much more speed you are looking for...want to do 60 all day? Or 70? At what kind of RPM?

  3. #3
    Join Date
    Feb 2009
    Location
    Huntington Beach, CA
    Posts
    22

    Default

    i would like to cruise on the freeway at 75 doing no more than 2800 rpms. as to keep the gas milage reasonable.

  4. #4
    Join Date
    Sep 1998
    Location
    North Central Wisconsin
    Posts
    11,520

    Default

    OK...

    With stock tires, that means you need:
    axle gears of 4.56, which is the lowest numerically the stock axles can take without a carrier change.

    AND a .83 overdrive.

    This equates to 2806 RPM (approx.) at 75 MPH.


    If you go to a true 38 inch tire:
    4.56 gears
    AND
    .93 overdrive
    results in 2813 RPM at 75 MPH.

    With a spring over and 39.5 in tires:
    4.56 gears
    AND
    .96 overdrive
    results in 2794 rpm at 75 mph.

    Without changing the axle gears:

    With a stock tire:
    .65 overdrive
    results in 2829 RPM at 75 MPH.

    With a true 38 inch tire:
    .72 overdrive
    results in 2804 RPM at 75 MPH.

    With a spring over and 39.5 in tires:
    .75 overdrive
    results in 2810 rpm at 75 mph.



    Then you HAVE to beef up the brakes over stock...at least in my book...stock brakes at 75, especially the single line master, wouldnt cut it.

    Handling on these trucks isnt that great either...speed like that would be a little nerve racking in my mind...for a long trip anyway...more than was intended stock...

  5. #5
    Join Date
    Dec 2006
    Location
    Auburn, AL
    Posts
    251

    Default

    I think this is a little more than you were looking at, but the vehicle (stock) was really never designed to do what you (and myself) want to do with it.

    The drive train is happiest at lower RPMS. Mine screams, leaks, and is generally unrully at higher speeds. There's a lot of reasons for the unrulliness, but it's not happy.

    If I were going to try to keep it sort of stock, I would change the gearing on the diffs and swap out the t-case for a 3:1 low. I am not a fan of the NP200. This solution requires a fair amount of work and not an inexpensive route, but I think you will be happier in the long run.

  6. #6

    Default

    Body swap, if high speed and daily driving is your thing. Even with brake upgrades and gear changes your still working with 40 year old equipment. Find a 97 f350 with the 460 and the auto and bolt your body to it.

  7. #7

    Default

    I have a similer setup. just got done putting on some 38x13 tsl. i only gained about 7 miles per our. 55 mph is it top speed. I am fine with that. But if you are wanting to go faster than 55 mph you will also have to chang your caster angle to. And put on a brake booster.
    68 M-715 460 Ford

  8. #8

    Default

    mine is not on the road, but i did some calculations for it. it will have 39s 4.56 rear end gears, and a .73 OD. i figured that i could cruise at 75 somewhere around 2100 rpm. i have done all of the other upgrades to make the truck less scary at those speeds. i have d60/14 bolt with disk brakes and power assist and dual circuit master. 2100 rpm is nice for a BBC.

    i am not an advocate for the body swap. i sy swap the newer driveline into the 715 if you want to get up to those speeds and make sure all of the equipment is safe for that kind of speed
    Ryan
    67 #18820 mostly stock...

  9. #9

    Default

    Quote Originally Posted by jeepcj View Post
    Body swap, if high speed and daily driving is your thing. Even with brake upgrades and gear changes your still working with 40 year old equipment. Find a 97 f350 with the 460 and the auto and bolt your body to it.
    Do an axle (modern 1-ton GM or Ford) and t-case swap (NP205) and you should be fine.

    My truck is quite happy with the cruise set a 75-80mph all day long. In fact, even at an observed top speed of 115mph it was still quite manageable (but not recommended with Michelin XZLs). Fuel economy suffers greatly above 45-50mph due to the flat windshield.

    That was with stock 715 body/frame, '85 GM 1-ton axles (D60/10.5"), 4-wheel discs, Saginaw PS w/ rock ram, mild-cammed EFI 460, 4R100 (E4OD) auto-od, Ford NP203/GM NP205 hybrid doubler setup & rear CV driveshaft from High Angle Driveline.

    My semi-annual $0.02...

  10. #10

    Default

    i cant imagine 115 in m-715 how was the pucker factor?

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