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Gearing for current day highway driving

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  • Gearing for current day highway driving

    Hi all - have an '86 totally stock drivetrain. Back in the '80's with the 55mph national speed limit, the factory SVO gearing was fine for the highway. Cruises along about 2500 RPM (maybe a bit less). Now that everyone is doing 75 (or more!) the car is cranking 3,000+ RPM. Aside from increasing noise it is just more wear on the engine. Has anyone modded their SVO to attempt getting the highway revs down? Rear end gears would change it but I don't want to sacrifice the off line experience. What about swapping the tranny with a 6 speed? I think there is a T56 or similar with 6 speeds but same concern - I would like the first 4 or 5 gears to have the same ratios as the stock '86 SVO and then just another higher, overdrive for 6th to get the revs down at 75mph.

    Any feedback on this appreciated.

  • #2
    I would imagine that a transmission shop could swap in a taller 5th gear. I don't have the ratio calculator in front of me, and our 86 is currently waiting for ANOTHER engine rebuild so I can't verify this, but I think 70mph was around 2500 rpm on our car.

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    • #3
      I had an 84 with stock rear end gearing and an 87 T coupe trans with the higher overdrive. I found it could not pull the gearing gracefully. It did not make the fuel mileage go up that much either. If you had a pimp or more power it might have been better but with the stock 84 175 HP it wasn't up to the gearing!
      Bruce

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      • #4
        Ok - I like the idea just changing out 5th gear. I suppose it would be a try it see how it works thing - if there would be issues with the gear being too much for the engine. The '84 gearing both in the rear and in the transmission is somewhat different from '86 and the '86 HP is higher so results could vary. Another thought this thread is sparking is possibly the rear end gear in mine was modified in the past by a previous owner causing higher revs. than it would be from the factory.

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        • #5
          I used an 84 SVO T5 in a former 80 Mustang with a naturally aspirated 2.3L, that was tuned up to ~140hp. I ran 3.27 gears in that car and it did just fine on the highway, but you did have to drop down to 4th if a quick pass was needed.

          Your 84 should have 3.45 gears in it, but they could have been swapped out for 3.73's or even 4.10's. You will need to verify the gears to see what you really have in there. That along with the size tires you are using, but it almost seems like you may have 3.73's.

          I am running 4.10's in my 86, along with slightly taller 245/45R17's and I hit 3000 rpm at ~72mph with the 86's 5th gear.
          Ted
          86 SVO Mustang
          17 Cooper S Clubman ALL4

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          • #6
            So now that I am home and have access to my calculator spreadsheet, here is some general info on what you should see for speeds and rpm with stock 225/50R16 tires:
            • 3.45 gears, 84-85 5th gear (0.81) --> 2600rpm/66.7mph; 2800rpm/71.9mph; 3000rpm/77.0mph; 3200rpm/82.1mph; 3400rpm/87.3mph
            • 3.73 gears, 84-85 5th gear (0.81) --> 2600rpm/61.7mph; 2800rpm/66.5mph; 3000rpm/71.2mph; 3200rpm/76.0mph; 3400rpm/80.7mph
            • 3.45 gears, 85.5-86 5th gear (0.78) --> 2600rpm/69.3mph; 2800rpm/74.6mph; 3000rpm/80.0mph; 3200rpm/85.3mph; 3400rpm/90.6mph
            • 3.73 gears, 85.5-86 5th gear (0.78) --> 2600rpm/64.1mph; 2800rpm/69.0mph; 3000rpm/74.0mph; 3200rpm/78.9mph; 3400rpm/83.8mph
            Looking at what I have on record for the various T5 gear sets, you have potential choices of 0.73, 0.68 & 0.63 5th gear sets, but not sure if they are compatible with your tranny gear set. Not sure if anyone on here has ever transplanted a different 5th into their case. First name I think of to call would be D&D Performance in Michigan, as they have been supporting the T5 world for many years and would likely know off the top of their heads what gears could be dropped into your non-World Class T5.

            Your 4.03 first gear can easily support a 3.27 rear gear set if you want to be more concerned with highway cruising than stoplight/drag racing. I was able to get down into the 15.9's with my 80 Mustang with the 84 T5/3.27 gear combo, but then again, that car only weighed 2640 lbs so less weight to get moving.
            Ted
            86 SVO Mustang
            17 Cooper S Clubman ALL4

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            • #7
              Looking at what I have on record for the various T5 gear sets, you have potential choices of 0.73, 0.68 & 0.63 5th gear sets, but not sure if they are compatible with your tranny gear set. Not sure if anyone on here has ever transplanted a different 5th into their case. First name I think of to call would be D&D Performance in Michigan, as they have been supporting the T5 world for many years and would likely know off the top of their heads what gears could be dropped into your non-World Class T5.
              I checked on this one time with Hanlon Motorsports and he said that only the 0.73 gear was compatible with my 86 T5 tranny.
              Mike S

              '86 SVO 9L Leather
              '86 SVO 9L Road Warrior
              '96 300ZXTT

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              • #8
                If Hanlon told you it is, you can believe it.
                Eric C
                SVOCA Webmaster

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                • #9
                  That is what I was kind of figuring, that you have to take the gears as 'sets' vs being able to plug-n-play with them. Hanlon is another T5 expert house for sure.
                  Ted
                  86 SVO Mustang
                  17 Cooper S Clubman ALL4

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                  • #10
                    While this discussion was ongoing, this image popped into my head!

                    Someone is always a smart arse. To make some contribution, I find the TREMEC Tool Box app handy to run any gear ratio, tire size, rpm, etc., to play the numbers, so to speak. T56 might be the ideal, if not expensive, solution. Had a 2012 Boss 302 with this trans, seemed bullet proof. However, with my brain, was too many gears to keep track, plus it was doing skip shift 1st to 4th just to irritate.

                    Here's good video of someone doing T56 swap in his S197, not sure what year. https://www.youtube.com/watch?v=B_CoBGfN5Gg

                    Bunch of good guys here to offer you advice.
                    You may only view thumbnails in this gallery. This gallery has 1 photos.

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                    • #11
                      yeah, there's some SVO's swapped with T56's around...
                      Eric C
                      SVOCA Webmaster

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