3:42 gears with a ST 300 2 spd on a 68 Skylark

Discussion in 'Got gears?' started by MEXICOYA, Oct 15, 2020.


    MEXICOYA Well-Known Member

    Has anyone installed 3:42 gears and posi with the stock ST300 2 spd transmission? I was wondering how high the RPM will be during freeway driving say at 75 MPH. I've also called a few tranny shops in my area they said that going to a TH350 3 spd will not make a difference on the RPM...true? I was also told going with a 700r4 is better than a 200r4 because parts are easier to come by if needed with the 700.

    I found a shop that can install posi and gears ranging from 3:08 to 3:90.

    Any input will be greatly appreciated.
  2. thebuick

    thebuick Well-Known Member

    I had the same setup 1968 custom lark w/350 mild mods, had 342 posi installed ran great ,until I took it to the Dyno shop, BUDDA BOOM , first pull tranny blew apart everyplace [ not good ] So I went to a 350 and that was the ticket. ran beautiful.

    MEXICOYA Well-Known Member

    Do you still have that now and how's it running? How high was your RPM when on the freeway?
  4. 1969BSGS

    1969BSGS Active Member

    PM Sent...
  5. No Lift

    No Lift Platinum Level Contributor

    Use this to calculate what your rpm will be. For best accuracy do a loaded rollout measurement of the rear tires and then go backwards calculating height. Or at least do a loaded height measurement.


    They are correct. Both ST300 and T350 have the same top gear 1:1 so no change in rpm. However the T350 has a 2.52 1st gear while the ST300 1st gear is 1.76(I believe). That means that a ST300 with 3.42's would have an overall effective ratio of 6.02 in 1st. A T350 with 3.42's would be 8.62, much better. However because of the better T350 first gear you could have 2.39 rear gear and have the same off the line ratio as the ST300/3.42 setup. That means you would effectively have an overdrive with those gears. With the T350 if your current gears are 2.73's you would have 6.88, 3.08's would be 7.76, both of which would be better than the ST300/3.42. So you can see switching to the T350 would be a better idea than the gear change. Plus, if you add a mild converter in there with maybe 2000-2400 stall, nothing expensive, you'll really pick up off the line. A mild converter works fine even on a stock, preferrably well tuned, engine. There may be some extra inefficiencey involved with the T350 because of the extra gear but not enough to make a noticable difference in the calculations.

    The only time the 2 speed even starts to get back in the game is when you really gear up the rear or are making some serious power. I had a mostly stock '65 GS with the ST300 but I took the 3.08's out and installed 4.56's in a 12 bolt. That thing used to fly off the line.
  6. pbr400

    pbr400 68GS400

    I once had a ‘79 Trans Am that had a T350 and 2.41 rear gears. It was like a 2 speed with OD; first and second gear were the launch and drive around gears, third was highway cruise. With a low compression stock 403 it would run 14.80 in the quarter in second. Third allowed high teens mpg and a top end well past the 100 on the speedo; 3000 rpm was 90.

    Be sure the shop knows what rear is in your car now. If it’s the original ‘68 8.2, they can’t just swap in gears...
  7. TrunkMonkey

    TrunkMonkey Well-Known Member

    Final drive is final drive (direct drive), and is always going to be the same for any transmission that does not have overdrive (or allowing for converter slip in an auto).

    Any good calculator for drive tire diameter, rear gear ratio and engine RPM will provide the answer.


    MEXICOYA Well-Known Member

    Thanks for the info!
    TrunkMonkey likes this.

    MEXICOYA Well-Known Member

    Yeah, I've been told it may be best to wait for Fabcraft's gear packages. Thanks for your response
  10. Bigpig455

    Bigpig455 Fastest of the slow....

    I have that set up - runs about 3200 at 75 with 26 inch tall tires
    philbquick likes this.
  11. monzaz

    monzaz Jim

    Overdrive is the only way your going to get your RPM down on the top end with steeper gearing.
    As stated 350TH and 400TH will only help on the low end
    It also pertains to the 4 Speed Manual trannys also. (stick shift.)
    All the extra is gear manipulation from shift to shift, NOT the top end ....It will still be 1:1 in 4th gear same as 350TH 400TH and your 2 speed trannys.
    JD race
  12. Mark Demko

    Mark Demko Well-Known Member

    3.42 is the tipping point for me with no overdrive on the highway, any deeper gear I'd go overdrive.
    IIRC, my rpm @ 70 mph is about 3 to 3200.
    Im sure my JW 9.5" plays into that also, it flashes to 3500 rpm.
  13. LARRY70GS

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

    Should be 336/26 X 3.42 X 75 = 3315 RPM, and that's without converter slip. All this depends on an accurate speedometer and tachometer.
    TrunkMonkey likes this.
  14. TrunkMonkey

    TrunkMonkey Well-Known Member

    3.5% variance is the standard at highway speed. :)

Share This Page