picking rear ratio

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  1. BB BELLA

    BB BELLA Well-Known Member

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    trying to pick a rear gear for the zf s6-40 which has double OD. car will see lots of highway and I like not constantly pulling gears on the street! what gear 3.91/ 4.10 or ?
    833 2.66/1.91/1.39/1.00
    zf 2.68/1.80/1.29/1.00/.75/.50
    500+ci alumhead 11/1

    IMG_0331.jpg
     
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    • 69L48Z27

      69L48Z27 Well-Known Member

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      3:73 to 3:91 would be my preferred range.
       
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      • 1 Wild R/T

        1 Wild R/T FBBO Gold Member FBBO Gold Member

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        Sorry but you've out cooled yourself... Your comment "I like not constantly pulling gears on the street!" but you've got a 6 speed trans...

        The reason modern cars are going to 6-10 speed transmissions is engines with limited torque & a narrow band where the engine is efficient...

        You on the other hand have 500CI of torque that does not really care what gear it's gonna pull...

        5th gear is .75 so a 3.73 or 3.91 gear would be fine... But 6th gear is .5... You've never need it

        You could jump to 4.56 so 6th gear is 2.28 but then 1st & 2nd are worthless unless your hauling a heavy load....

        Yup, plain & simple you've out cooled yourself..
         
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        • Jerry Hall

          Jerry Hall FBBO Gold Member FBBO Gold Member

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          3.91 is my choice. It has great performance 1-4 and right around 2000-2200 rpm at 75-80 on the interstate. In contrast my 66 with 6.i Hemi and 727 is 3500 at 68-70 with 3.55 gears.
          Torque and HP is what is going to keep down rowing gears.
           
        • Slap Stick

          Slap Stick Well-Known Member

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          I think what was meant by, "pulling gears" was referring to the 3rd member.
          Do the math using tire diameter, etc to find engine rpm at the desired speed in OD.
           
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          • Hey-O

            Hey-O FBBO Gold Member FBBO Gold Member

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            I thought he was talking about changing gears in the rear end also. I would go 3.91
             
          • R413

            R413 Well-Known Member

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            Info not disclosed:
            -rear tire height 25” or 31” ?
            -desired RPM, cruise and top speed?

            Are you considering driveshaft RPM?
             
          • toolmanmike

            toolmanmike Moderator Staff Member FBBO Gold Member

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            Ya with 500 cubes you could get buy with a 3 speed and 2.87's.
             
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            • Slap Stick

              Slap Stick Well-Known Member

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              Driveshaft rpm I think is irrelevant. Its engine rpm at x mph in high gear. Don't over think it.
               
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              • Budnicks

                Budnicks You Can Just Thank Me Later FBBO Gold Member

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                http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php

                1 3.73 gear
                in 6th with a 0.50% OD
                with a 28" tall tire it's like
                @ 65 mph it's like 1,455 rpm
                @ 70 mph it's like 1,567 rpm


                a 3.91 gear same other stuff
                @ 65 mph it's 1,525 rpms
                @ 70 mph it's 1,642 rpm

                a 4.10 same other stuff
                @ 65 mph it's about 1,599 rpms
                @ 70 it's 1,722 rpm

                or try other calculators there to figure out what you need

                good luck
                 
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                • BB BELLA

                  BB BELLA Well-Known Member

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                  .
                  thinking 3.91 is the gear!Don't forget I'm in Canada and it's quite common for traffic to be moving 130kph+19kph before dangerous driving!that's 92mph.
                  1wild r/t is correct about my not wanting to row gears. now that I see the term "rowing"! I prefer longer hard pulls that keep you back in the seat instead of jostling around in the car!
                   
                • Hey-O

                  Hey-O FBBO Gold Member FBBO Gold Member

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                  Well done!!
                   
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                  • BB BELLA

                    BB BELLA Well-Known Member

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                    forgot to mention this tranny was sitting in the shop destined for the 6/71 blown gen6 big block 80 2 wd blazer with much more tire , my son decided it might not be tough enough for the truck and s/c motor so it goes in the mopar!picked up a richmond 5 speed for the blazer.
                    It'll all be together soon and we'll see if we know anything about anything!maybe we'll have a lot of scrap!
                     
                  • Cranky

                    Cranky Banned Staff Member FBBO Gold Member

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                    With a .5 OD.......4.10's at least and being me, 4.30's! Geez. Do the math with the 4.10's and a .5 OD and see what your rpm is at 90 mph. Heck, I've run 4.56's on the street and some short highway stuff with a small block and no OD! Yeah, 60 MPH is getting up there but for short hops, no big deal. And no one and I mean no one took me off the line! :D
                     
                  • BB BELLA

                    BB BELLA Well-Known Member

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                    it's not tubbed!keeping the tires hooked is #1 in my opinion! these cars won't fit much rubber under them! that's why I'm thinking longer pulls with less tire contact disruption!
                     
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