Rpm at highway speeds

Discussion in 'The "Juice Box"' started by white72gs455, Aug 9, 2014.

  1. white72gs455

    white72gs455 Going Fast With Class!!!

    This may be a silliy question...
    What should the RPM's be at while cruising at highway speeds? Say 55 mph?
    I know things that effect it are how things are built, I'm just wondering what it should be near...
    Mildly built 70 455 with stage 1 heads, TH400, 3:42, 26.6" tall tires.
    The trans does shift out quick under normal driving(1,2 & 3) and under WOT.
    I was just thinking it was high while on the highway.
    Maybe it's just because of the headers and H pipe 3" pipes.

    Let the hazing begin!
    :grin:
     
  2. Brad Conley

    Brad Conley RIP Staff Member

  3. killrbuick66455

    killrbuick66455 Well-Known Member

    Hey white72gs455, your rpm should be at 2700 at 60 MPH with running a 26 tall tire your actual gear ratio is around 360 range
     
  4. white72gs455

    white72gs455 Going Fast With Class!!!

    Thanks! You guys are great!
     
  5. Nothingface5384

    Nothingface5384 Detail To Oil - Car Care

    whatever you want it to be...comfortable with(if your talking about regearing)
    I like to cruise at 70-75mph on highways so I gear my car to be at 3k around that mph
     
  6. 72newbiebuick

    72newbiebuick Gold Level Contributor

    So let me chime in here....

    Unsure honestly (ugh) if I put in 3.08 or 3.23 gears in my 72 Skylark rear - no didn't jack it and spin the wheel yet. I have 28" tires and run my TH350 around 3k rpm at 60mph or so. Really contemplating the 200r4 for the overdrive and assistance off the line. Concerns about me running too low rpm at highway speed based off what I mentioned?

    Mark
     
  7. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member


    336/tire height X rear gear X MPH = RPM without converter slip.

    336/26.6 X 3.42 X 55 MPH = 2376 RPM. Add maybe 100 RPM for converter slip, and you are at 2500 at 55 MPH.
     
  8. white72gs455

    white72gs455 Going Fast With Class!!!

    Thanks again!
    I guess I need to hook my tach back up, I disconnected it when I put in the conversion kit
    (MSD or is it the Crane unit? I'll have to look)
    Anyhow I disconnected it for a reason, but I don't remember why...
    Does the tach go to the negative side of the coil, same as with points?

    Sorry I have been working on the 54 and haven't paid much attention to the 72.

    Thanks
    Jerry
     
  9. 70 gsconvt

    70 gsconvt Silver Level contributor

    My car has 28" tall tire, a 3.73 rear gear and a 200-4R trans. At 70 mph, I'm cruising along at around 2200 rpms.
     
  10. 72newbiebuick

    72newbiebuick Gold Level Contributor

    so if I have a 3.23 gear instead of 3.73 any idea what rpm I might be running at 70mph?

    Mark
     
  11. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    The 2004R has a .67 OD. .67 X 3.23 = 2.1641

    336/28 X 2.1641 X 70 = 1818 RPM in lock up.

    the tachometer lead goes to the - side of the coil. The two exceptions I can think of are the GM big cap HEI (tach terminal on distributor cap), and an MSD or other box (box has a dedicated tachometer output terminal)
     
    Last edited: Aug 11, 2014
  12. 72newbiebuick

    72newbiebuick Gold Level Contributor

    1818 rpm..... might be too low? Any way to adjust shift points?

    mark
     
  13. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Why? My 602 HP roller motor has no problem at all with 2000 RPM at 55 MPH. I have a GV overdrive. It also has no problem with lower RPM in stop and go traffic. Unless you have a huge cam , it isn't too low.
     
  14. 72newbiebuick

    72newbiebuick Gold Level Contributor

    Hey just checking what is best for power band ranges... if the lower rpm is good to go then I am changing it out!


    Mark
     
  15. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member


    Might you want to tell us what cam you are running and the rest of your combination?
     
  16. 72newbiebuick

    72newbiebuick Gold Level Contributor

    Sure thing...

    350 bored .030 over with comp cams b268 mild cam (.469/.469), cast iron 4bbl intake, Quadrajet for my year provided by Ken, 10:1 (or so compression), 3.23 (I believe) posi in back, TH350 (right now).

    Mark
     
  17. mikec

    mikec Well-Known Member

    I am running a 3.23 gear with 27.7" rear tires and I turn 2500 rpm at 70. Speed measured by GPS and rpm measured by the factory console tach which might be +/- 200 rpm.
     
  18. bobc455

    bobc455 Well-Known Member

    I LOVED my 2.56 rearend behind the 455.

    A 2.56 might not work in a high-revving chevy, but it works great behind a 455
     

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