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79 T-Bird best gear ratio

Discussion in '1967 - 1988 Ford Thunderbird' started by chgordon, Nov 16, 2018.

  1. chgordon

    chgordon New Member Gold Donor

    Hey just bought a beautiful rust free 79 T-Bird (with the super rare bucket seats / centre console option!). Previous owner rebuilt original 302 - added a 4 barrel carb, headers, Flowmaster exhaust etc. He swapped the C6 trans for a lighter C4 with shift kit. All good stuff. Diff gear ratio is 3:8-1. The car has great acceleration but on the highway the low gearing is a problem. At 60 miles per hour the motor is revving about 3,000 RPM - way too high. I wonder what a good gear ratio would be - the motor is good for about 250 HP. Original ratio was about 2.5:1. I want to cruise on the freeway at 70 and have the motor turning at about 2500 RPM. On the other hand it's a heavy car and I don't want it to feel sluggish. Any thoughts? Thanks guys :)
    DEVIL DOG 62 likes this.
  2. Nothing you can do about gear ratio. Even a lower ratio like 2.80 will scream on the highway. C4 or C6, both are three speed transmissions. Also the fact you got a 302 in that BIG car is another reason you are reving so high. Takes a lot of that Engine to push that boat along. Only thing I can recommend is another transmission. Something with more gears or overdrive will cut down on the revs when on the highway. Other than that you are pretty much out of luck.

    Back in the 60's and 70's highway speeds were not like they are today. My 1969 tbird has the same issue. Anything over 60ish mph and the engine will rev pretty high to maintain the speed. It's just the issue with the transmission and the fact its only a three speed.
    Last edited: Nov 16, 2018
  3. chgordon

    chgordon New Member Gold Donor

    Thanks for your input. I'll probably switch the rear end gears anyway - I think 3.8 is just too low. It would be a shame to replace the trans as it has just been rebuilt.
    DEVIL DOG 62 likes this.
  4. Changing the rear isn't going to do very much. My 1969 has a ratio of 2.80 which is a ratio designed for highway cruising. Even then like I said anything over 55-60 and the engine revs pretty high. You don't want to go any higher either. You will just get even worse fuel economy. Sorry dude but these transmissions are just not designed for highway travel on modern highway speeds. You could end up putting unnecessary strain on your transmission going with a higher gear ratio. Just trying to help. I was wanting to do the same with mine and this is pretty much your only choice.
  5. Also the other problem is with your year. Is most of the emission stuff still connected? These cars were really de-tuned and hampered by emissions in the mid to late 70's.
  6. chgordon

    chgordon New Member Gold Donor

    No - all the smog stuff is gone. The previous owner rebuilt the 302 and it has a 4 barrel carb - it's pretty stripped down to the basics. I was thinking of changing to like a 3:1 gear ratio - close to the stock one like yours has. I think it would improve the high revving somewhat - fuel economy not a huge deal... Mostly want a quieter ride on the highway and less strain on the engine.
  7. Where they still putting 9 inch rears in your car? I am wondering because mine now has a 460 with the C6 transmission and 9 3/8 inch rear (pretty much a 9 inch). If it is a 9 inch you got some options on what you want.
  8. chgordon

    chgordon New Member Gold Donor

    Yes it's a 9 inch. Seems like there's a ton of options. I'll talk to my local shop about it - they seem to know a lot about these things. All I know is the 3.7 gear that's in the car now is not good - the stock ratio was something like 2.5. I imagine much lower revs at highway speeds with that. Maybe something between the two would be good.
  9. You won’t get much lower at 70mph. Maybe 60ish, but 70? Nahh... not going to happen. It’s the main reason I myself stay off the highway and just stick with regular roads and such.
  10. You could also try changing the gearing in the trans itself. Problem I see with the C4 or C6 is the gears are very close together with the final being a very long.
  11. Sounds like previous owner souped it up a bit and didn't do too much highway driving but if running too high RPM's for your highway driving just put the original 2.5:1 ratio back in it. Unless you're planning on racing it in 1/4 mile tracks
  12. chgordon

    chgordon New Member Gold Donor

    Thanks - that's exactly right. Original ratio will be way better for highway driving. Previous owner a bit of an old school hot rodder. Not really my thing.
    DEVIL DOG 62 likes this.
  13. I use a 1979 Thunderbird as my daily driver here in Florida. It has the factory 302 and the factory 2.73 gear ratio in the rear. At 70 mph, it does rev a bit, but still very acceptable even with the windows down on cool day. Perhaps you could swap out a similar rear end from a 79 Bird in your local scrap yard.
  14. chgordon

    chgordon New Member Gold Donor

    Thanks for the info. I think I'm just gonna go with the factory gear ratio. Is it 2.5 or 2.73? I have a 302 rebuilt with a 4 barrel - headers etc. Seems pretty peppy. Everything in the diff/rear axle is new (brakes, bearings etc.) A ring gear and pinion switch should do the trick.
  15. Have you tried getting a Martiauto report? It can tell you everything you want to know about your car. It's a fascinating thing. I did one for my car. Here check it out! Oh and heres the link too.
    ChuckH262 likes this.
  16. chgordon

    chgordon New Member Gold Donor

    Wow that's very cool! I will definitely get one for my 79. Thanks for the info - I had never heard of such a thing. It's like a birth certificate for our T-Birds!
  17. Sierra977

    Sierra977 Well-Known Member Lifetime Donor

    Why would you buy a car that's been hot rodded and then start destroying the car? 3.78:1 rear axle ratio is about as perfect as it gets for everyday driving. If freeway driving is your plan why did you buy a hot rod in the first place?

    Here are factory axle ratios for 1979 Thunderbird

    1 2.50:1
    B 2.47:1
    C 2.47:1 Traction-Lok
    J 2.50:1 Traction-Lok
    6 3.00:1 (400 V-8 only)
    O 3:00:1 (400 V-8/Traction-Lok)

    C4 is a three-speed automatic. Replace transmission with an overdrive unit.
    OneAmongOthers likes this.
  18. This I agree with. It's the transmission that's the problem. Three speeds just isn't enough. There is a few options for a transmission with overdrive out there that require little modification.
  19. chgordon

    chgordon New Member Gold Donor

    Holy cow judgemental much??? Destroying the car?? Just looking for some friendly advice bud. Lighten up.
  20. Admittedly a bit harsh, but there is truth in his words. You just won’t get decent highway cruising on modern highways with the C4 transmission. It’s one of the reasons it was removed from passenger cars as well as the C6. They just are a relic of past times when highway speeds were not what they are today

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