Highway driving....RPMS

Engine, Trans & Driveline

  1. koosh

    koosh Well-Known Member

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    So, in today's world, if your not doing 70-75 on the highway, you get run over....my 69, 383, 4 speed car pretty much screams at those speeds.....does that mean I have 3.55 gears? (Or higher)? Is there simple way to find present gear ratio? If wanting highway cruiser, what should I be running?
    PS....running 295/45/18 on rear
    Thanks
     
  2. Bad Sport

    Bad Sport FBBO Gold Member FBBO Gold Member

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    Jack up the rear and place jack stands under the axle tubes. Put a mark on the driveshaft, put a corresponding mark on the pig. Put a mark on the ground/floor by the tire and a corresponding mark on the tire.

    If it's an open rear turn the tire one complete turn while counting the number of turns on the driveshaft.

    If it's an open rear with one tire on the ground turn the tire two complete turn while counting the number of turns on the driveshaft.

    If it's a suregrip do one complete rotation while counting how many times the shaft turns.

    For instance, if it turns 3 1/2 times most likely 3:55's, 3 1/4 turns most likely 3:23's..........if it turns just shy of 4 turns most likely 3:91's.

    I run 3:55's and at 60 I'm turning around 2600-2800 rpm's, with a 26" tall tire.

    Running a taller tire will effectively slightly lower the gear ratio number numerically.
     
    Last edited: Feb 25, 2019
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    • koosh

      koosh Well-Known Member

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      Thanks!
      I'll have to check rpms at 60, but highway's gonna be at least 70mph
      How do I know if open or sure grip?
       
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      • Bad Sport

        Bad Sport FBBO Gold Member FBBO Gold Member

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        Typically an open diff when rotating the passenger tire forward the driver side will rotate opposite, sure grip both in the same direction.
         
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        • koosh

          koosh Well-Known Member

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          Thanks!
           
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          • Ranger16

            Ranger16 Well-Known Member

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            • 451Mopar

              451Mopar Well-Known Member

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              Your tire diameter should be about 28.45".
              At 75 MPH, 5.57:1 gear = 4,933 RPM
              At 75 MPH, 5.17:1 gear = 4,579 RPM
              At 75 MPH, 4.89:1 gear = 4,331 RPM
              At 75 MPH, 4.56:1 gear = 4,039 RPM
              At 75 MPH, 4.30:1 gear = 3,808 RPM
              At 75 MPH, 4.10:1 gear = 3,631 RPM
              At 75 MPH, 3.91:1 gear = 3,463 RPM
              At 75 MPH, 3.73:1 gear = 3,304 RPM
              At 75 MPH, 3.55:1 gear = 3,144 RPM
              At 75 MPH, 3.31:1 gear = 2,932 RPM
              At 75 MPH, 3.23:1 gear = 2,861 RPM
              At 75 MPH, 2.93:1 gear = 2,595 RPM
              At 75 MPH, 2.76:1 gear = 2,444 RPM
               
            • moparnation74

              moparnation74 FBBO Gold Member FBBO Gold Member

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              It will scream at 70 mph with a 4spd, tall tire and 3.23 and bigger
               
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              • koosh

                koosh Well-Known Member

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                Are you recommending a particular ratio?
                 
              • moparnation74

                moparnation74 FBBO Gold Member FBBO Gold Member

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                Less...or go with a 5 speed....

                My X 3.54 dana 4spd would scream at 70 with stock tires......
                 
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                • kiwigtx

                  kiwigtx International Mod Staff Member

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                  Glad you asked....I conducted this very experiment on my buddy's driveway a while ago...

                  Suregrip marks.....
                  upload_2019-2-25_6-12-25.png


                  and a Pegger......
                  upload_2019-2-25_6-12-44.png


                  Hopefully that concludes today's lesson. :D
                   
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                  • Dajkzo

                    Dajkzo FBBO Gold Member FBBO Gold Member

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                    One of the few down sides of a vintage Mopar. The short list of options for overdrive. I have a used GearVendors that I plan on putting in. However with 3.91, it will still be turning 2700 @ 70 mph. Right now it's 3500 and I have to stay out of the left hand lane or an SUV will run me over. LOL
                     
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                    • 69 GTX

                      69 GTX FBBO Gold Member FBBO Gold Member

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                      Gear vender should put you at a 3.04 ratio or about 2700-2800@75mph
                       
                    • Grabinov911

                      Grabinov911 Well-Known Member

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                      • koosh

                        koosh Well-Known Member

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                        Is it not cheaper just to change rear end gears?
                         
                      • Ranger16

                        Ranger16 Well-Known Member

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                        I forget my exact tire diameter, but they are on 15' Cragars.
                        '69 4 speed with a 3.23 rear end.
                        My RPM is as follows.
                        50 MPH = 2000 RPM
                        55 MPH = 2200 RPM
                        60 MPH = 2400 RPM
                        65 MPH = 2600 RPM
                        70 MPH = 2800 RPM.
                         
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                        • 747mopar

                          747mopar FBBO Gold Member FBBO Gold Member

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                          Of coarse it is but then the car isn't quite as much fun either loosing a lot of that snap.
                           
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                          • khryslerkid

                            khryslerkid FBBO Gold Member FBBO Gold Member

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                            Looks like what I have with 3:23s, 275/60/15 Coopers with a Torqueflight
                             
                          • 33 IMP

                            33 IMP Well-Known Member

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                            Bad sport is mostly right, but you don't multiply by two with a suregrip. Any limited slip diff will act like a spool if it's any good at all, if the tires are in the air, so simple observation of the number of turns is correct.
                            If an open rear end, with one tire on the ground you must change the number of turns because of side gears and spider gears in the diff. It's easier just to jack up both, then they will both rotate (in opposite directions) and again count turns of the driveshaft for one turn of either tire.
                             
                          • Kern Dog

                            Kern Dog FBBO Gold Member FBBO Gold Member

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                            OR you look for a tag on the mounting nuts of the third member....OR remove it to know for sure.
                             
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