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C2 Rear End ratios

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  • Mike Iachelli

    C2 Rear End ratios

    Finally got my 64 coupe on the road. It has the 365 hp engine, 4 speed tranny and a 4:11 rear end. Intend this to be a driver and, being in California, this means freeway driving with the engine running aproximately 3500 rpms in regular traffic. Bit too high for me in long distance travel. Thinking of swapping the rear end gears for a better cruiser. Do not know the formula but believe would like to be able to cruise at 70 mph at around 233-2500 rpm's while still having a strong pulling car from a standing start.

    Any suggestions for which gear ratio would suit my purpose? And, if so, what do we do to correct the speedometer?

    Thanks
  • Jay G.
    Expired
    • August 31, 1993
    • 398

    #2
    Re: C2 Rear End ratios

    Great gears 3:31 to 3:55. A Nice Top end gear 3:08. Just put a 3:36 gear in my 69 Nova SS 396 L-78 went from 4:10 makes for a nice cruiser with better Gas milage. Cars at 2100 rpm at 65 mph on 14" redlines.

    Comment

    • Wayne K.
      Expired
      • December 1, 1999
      • 1030

      #3
      Re: C2 Rear End ratios

      Mike,

      With a close ratio transmission (if that's what you have) I personally won't go higher than 3:55 with the engine you have. A nice combo if your looking for lower RPM than what 3:55 will give you would be to go to a wide ratio tranny with 3:36 gears.

      Wayne

      Comment

      • Tom Merkel

        #4
        Re: C2 Rear End ratios

        Mike - A 3:55 will give 2400 rpm at 60. Good middle-ground ratio. Plenty of pep; nice top-end cruise.

        Tom

        Comment

        • Don 42616

          #5
          Re: C2 Rear End ratios

          I had a 65. Changed a 4.11 to a 3.36. Better on freeways.

          Comment

          • Jack H.
            Extremely Frequent Poster
            • April 1, 1990
            • 9906

            #6
            Re: C2 Rear End ratios

            Advice others have given you regarding rear ratio to meet your objective (3:08-3.55) is solid. BUT, remember you have a 4-Series rear end and going lower (into the 3.xx range) means swapping the whole carrier assy! Then, you have to drop the 4-speed and change drive gears inside to match your new rear end ratio to get the speedo/odo to correctly match...

            That's a LOT of labor and parts changing, plus you probably should have done this when the car was previously apart! On the tranny gear, you can 'poor boy' the task by having a speedo shop build you a custom external gear box to compensate for the new rear gear ratio. But, it'll be an external and visible modification...

            Comment

            • Gerard F.
              Extremely Frequent Poster
              • June 30, 2004
              • 3806

              #7
              Re: C2 Rear End ratios

              Mike,

              A 3:08 would be right around your spec of rpm vs speed. You would have to change the speedometer gear at the transmission but you will never get exactly right on due variables in tire size the increment in the speedometer gears.

              You can calculate a correction factor for your speedometer by counting post mile markers and comparing the count with your trip odometer readings. I did this with my 67/3.08 for 100 miles going to Flagstaff, and came up with a correction of +2.5%. Arizona has some great post mile markers, not like the little dinky ones in CA.

              Jerry Fuccillo
              #42179
              Jerry Fuccillo
              1967 327/300 Convertible since 1968

              Comment

              • Duke W.
                Beyond Control Poster
                • January 1, 1993
                • 15676

                #8
                Re: C2 Rear End ratios

                With a total ratio spread of 2.2, you can't have your cake and eat it too. Your current setup is like a five speed with no fifth. Go to a tall gear like a 3.08 and it's like a five speed with no first. Take your pick, and remember that you have a torque shy engine, so tall gears could be somewhat painful to drive in low speed or stop and go traffic and clutch life might be short. A 300 HP with a wide ratio four-speed and 3.36 or 3.08 would be a much more pleasant combination.

                Revs at 60 are 775 x axle ratio, so with a 3.08 you would be turning just under 2400 at 60. You can do the math for other ratios and linearly proportion to compute revs at other speeds. If you wanted to drop revs to 2300 at 70 you would end up with a 90 MPH first gear, which would be really tough on clutches and doggy around town to say the least.

                Consider buying a Richmond wide ratio Super T-10 (2.64. 1.75, 1.34, 1.00:1) combined with a 3.36 axle. This would give you about the same overall first gear with about 3000 revs at 70 - a decent all around combination for modern driving. If the installed trans is original, set it aside and save it, but include it for the next owner if you sell the car. If it's not the original trans, just trade or sell it.

                Duke

                Comment

                • Louis T.
                  Very Frequent User
                  • July 31, 2003
                  • 282

                  #9
                  Re: C2 Rear End ratios

                  Can you elaborate on why one would have to replace the entire carrier assembly? Are you saying that one can't simply swap the 4.11 gears with, e.g., 3.36 gears in the rear? If so, what is different internally? Also, how many different carrier assemblies are there with respect to the gearing range (e.g., only two - 3.xx and 4.xx?) and how does one distinguish them externally, if at all possible?

                  Also, regarding the transmission "adjustments," I understand there are two different speedo steel driven gears, one covering 3.08-3.55 and another 3.70-4.11, and if one crosses over, e.g., from a 4.11 to a 3.36, one will have to change the speedo steel driven gear from the latter to the former along with changing the speedo drive gear from a 24-tooth (4.11) to a 20-tooth (3.36). Is this correct, and all that is required with respect to the transmission?

                  Thanks in advance.

                  Comment

                  • Duke W.
                    Beyond Control Poster
                    • January 1, 1993
                    • 15676

                    #10
                    Re: C2 Rear End ratios

                    Jack meant to say "case" not carrier. The differential case is the internal casting that holds the differential gears, clutch pack (if Postraction) and ring gear. The "carrier" is the external housing that holds the case assembly and drive pinion.

                    There is a spacer you can use to install a 3-series gear on a 4-series case, or maybe it's the other way around, and and IIRC the internal speedo drive gear is the same up to 4.11, so an axle ratio change can be accomodated by changing the externally accessable driven gear to on tooth count matched to the axle ratio.

                    Duke

                    Comment

                    • Phil P.
                      Expired
                      • April 1, 2006
                      • 409

                      #11
                      Re: C2 Rear End ratios

                      mph X gear ratio divided by tire size X 336 = rpm @ given mph
                      there are four variations of this formula to figure out rpm,gear ratio, tire size,and mph on the above formula simply plug in 70 mph and vary the gear ratio until you get the desired (rpm) result

                      Comment

                      • Jack H.
                        Extremely Frequent Poster
                        • April 1, 1990
                        • 9906

                        #12
                        Thanks, Duke! *NM*

                        Comment

                        • John H.
                          Beyond Control Poster
                          • December 1, 1997
                          • 16513

                          #13
                          Re: C2 Rear End ratios

                          To get 2500 at 70mph, you'd need a 2.87 rear end, which would be the slug of the century, regardless whether you had a wide- or close-ratio 4-speed. If you have a 4.11 now (which uses a Series 4 Posi carrier), any replacement ratio below 3.90:1 will require a new Series 3 Posi carrier ($450), plus the cost of the new ring & pinion, plus labor. There's no free lunch - the only way to get the freeway cruise rpm you want and decent around-town performance/driveability, you need a 5-speed with an overdrive top gear.

                          Comment

                          • Michael H.
                            Expired
                            • January 29, 2008
                            • 7477

                            #14
                            Re: C2 Rear End ratios

                            That sounds expensive. I'm sure glad I have a 3.70 in my 66 BB car. My favorite ratio, near the middle of the range. Perfect for around town but still not terrible on the highway.

                            Comment

                            • Tom S.
                              Extremely Frequent Poster
                              • March 1, 2004
                              • 1087

                              #15
                              Re: C2 Rear End ratios

                              I changed my 69 with a m-21 from 4:11 to 3:36 and it works perfect for the highway. A little bit of a slug on take off, but that was the std. gear for those car's.I am happy with the change. But I would have preferred a 3:55 I think that would be perfect. Tom

                              Comment

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