From 2.56 to 3.42 gears, mpg/rpm difference?

Discussion in 'The Bench' started by buicks, Oct 11, 2012.

  1. bw1339

    bw1339 Well-Known Member

    This is an interesting discussion. In my '66 Toronado (mild 455/TH425) I have the stock 3.20 gears. On the highway I find myself wishing for lower RPMs and during normal driving, I'm in 3rd gear 90% of the time. Even climbing a steep hill at 25MPH the car will do it in 3rd gear and has the torque to do it.

    I am in the process of swapping a 2.73:1 final drive into my car. My hope is to make it a better highway vehicle and -getting more use use out of the other gears - not lose much overall performance. Additionally, my car has a switch pitch torque converter, which I've never gained much from. In its current configuration, high stall just gave me slippage, so I found that taking off in low stall worked better for me, as the car was more responsive and required a lot less throttle to get going. It was as if I was getting slippage rather than torque multiplication. I've had two different S/P TH425s in my car and they both acted exactly the same. I'm hoping that by going with the taller gears, I can begin to get the benefits from the switch pitch torque converter.
    Thanks.
     
  2. gsgnnut

    gsgnnut Well-Known Member

    I used to get around 15 mpg highway in a 1970 GS350 with a peg leg 2.56 rear. when I put in a 3:42 back in mileage went down to around 10-12. These toys were not meant for 55mph, they were also not meant to be fuel efficient either so put in the ratio that gives you the biggest grin for the buck. I got a 3.33 in the GS 455 and it gives maximum grins from a stop light and about 8-10 mpg highway with over 3K rpms at cruising speed (and as a bonus it sounds bad ass ), but your mileage and grin factor may vary based on actual driving conditions.
     
    Jerry68GS400 likes this.
  3. Jerry68GS400

    Jerry68GS400 No Daddy, my GS

    That is the answer I was looking for. If the best we can get is around 15-18 mpg with the 2.56, then I may have to re evaluate what I will spend my money on. I like grinning. :) Does anyone else run gears like these (2.56s 2.73s etc) with a mostly stock 350 or 400? And is that about what we would get on the highway is around 15mpg?
     
  4. Jerry68GS400

    Jerry68GS400 No Daddy, my GS

    I appreciate all the hard work and experience and engineering it takes to really design a drivetrain. And as we continue to “restore” our Buick’s, I see how difficult it is to really know what you are looking for if you are inexperienced like we are. Which is why I joined this forum to pick the great minds and try to form a good guess of which direction we want to go. Thanks for all the help Guys and Gals of the muscle cars.
     
  5. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    Geez you guys get some lousy mileage. I'll give you guys my experience here. Way back when i had the original 2.56 rear in my skylark with the stockish 430 and the th350. It would get 15 all day on the highway. I swapped in the 4 speed and 3.42s. Along the way came a b4b, ported heads and headers. Oh and a gs112? Cam. It would turn 3400 or so going 75. It would pull 14 mpg on the highway. It would creep up to 16 country driving. I once drove it to Pittsburg 12 hours each way. The way home i held 80-85 and it was spinning 3700-3800. I got 12.5 doing that. It used a quart of oil. You guys have no reason to be scared of showing these engines some rpm. if your tune is right you have nothing to worry about. They are not time bombs.

    The stage 2 motor with 3.42s got 13 going 75. 15 country driving 50-55. That ran a very traction limited 12.05@115 too. I'm not sure how you guys are getting 7-10..
     
    Last edited: May 14, 2018
    300sbb_overkill likes this.
  6. PGSS

    PGSS Gold Level Contributor

    I had 3.23's in my 66 GS back in the day's and can't remember being bothered with excess rpm's on the highway. It pegged the speedo by if I remember correctly at 5100 or 5200.
    Didn't really need much rpms after getting to speed say at 65 as the weight of the car seemed to keep it going. How much does anyone think that the low stall of the SP300 had to do with the comfy rpms?
     
    Last edited: May 14, 2018
  7. Jerry68GS400

    Jerry68GS400 No Daddy, my GS

    I was really hoping someone would say “yeah, I swapped in a posi with 2.56 gears and I was getting around 30-35mpg with a mild built 400”. ;)
     
    johnriv67 and PGSS like this.
  8. TrunkMonkey

    TrunkMonkey Well-Known Member

    And low 13s in the quarter on 87 pump gas! :D
     
  9. LARRY70GS

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

    I've heard of even worse mileage than that. I'm thinking big Fat Holley Double Pumpers, and no vacuum advance.:D
     
    BeatersRus likes this.
  10. 300sbb_overkill

    300sbb_overkill WWG1WGA. MAGA

    If you want to max out the mpg with a 2.56 rear gear then you need to match the engine to that by making it as efficient as you can.

    That means matching the cam to the flow of the heads using the highest lift lowest duration @ .050" with the highest static compression you can run using premium fuel. A lower than stock stall speed would also help by limiting slippage.

    A better way would be to run an overdrive trans with a lockup converter with the engine setup the same matched to the proper gear to for the O/D trans. To figure out what the minimum gear for your O/D trans of choice would be to get the 1st gear multiplication ratio as close to 9.5:1 as you can, this will enable you to shift into O/D by 45 mph without lugging the engine down.

    You wouldn't think using the 1st gear multiplication ratio to chose a gear to run an overdrive trans would work so good but the math works outs great every time!

    So basically you would want at least a 3.42:1 rear gear with a 200R4;

    2.74(the 200's 1st gear ratio)x 3.42 = 9.3708:1 first gear multiplication ratio.(ratios will vary slightly with tire size and with a larger engine with higher torque, the gear can be reduced slightly and still be ok)

    Final drive ratio will be;

    .67 x 3.42 = 2.2914:1 which if you do the math should be in the RPM range you can spin the engine without lugging it down lower than it idles @ 45 mph.

    The bonus setting the car up this way is much more smiles per mile than running the punk azz 2.56 gear with a lower than stock stall. Really good off the line acceleration and very low highway RPMs.
     
  11. Jerry68GS400

    Jerry68GS400 No Daddy, my GS

    An overdrive transmission. 200R4? What used vehicles are those in?

    When you say at least 3.42, do you mean between 3.42 and 2 something or 3.42 and 4 something?

    And.... is it as easy as locating a 3 series posi unit and just installing it in an original open wheel differential?
     
  12. LARRY70GS

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

    The THM2004R was never designed to take big block torque in a heavy vehicle. It has to be beefed up substantially to last in a heavy performance vehicle. That does not come cheap. That transmission was used in a lot of GM vehicles over the years including the Buick GN, Olds 442, Monte Carlo SS, and other cars as well. That transmission had a 4th gear overdrive ratio of .67, and used a lock up torque converter. It also had a lower ratio 1st gear of 2.74. The whole purpose of using an overdrive transmission is so you can select a higher numerical rear gear ratio for better acceleration, but have the 4th overdrive gear bring it back down to lower the RPM on the highway. It's easy to figure out the final drive with simple multiplication. If you selected a 3.42 rear gear, the final drive in 4th gear OD would be .67 X 3.42 = 2.2914. If you use the 4th gear OD gear with a numerically lower rear gear ratio, you can make that gear unusable because it lowers the engine RPM too much at normal highway speeds.

    Your 68 GS400 has the Buick only 8.2 10 bolt rear. Aftermarket gears, posi units, and parts are not readily available. There was an aftermarket company that reproduced gear sets in 3.42 and 3.64, but I am not sure if they are still available. Your 8.2 rear is NOT the same as the 64-67 BOP 8.2 10 bolt. Parts are readily available for that rear in the Sumitt and Jegs catalogs. Don't confuse the 2 rears, it is a very common mistake to do so. It is not as easy as simply finding a posi unit and installing it in your rear. It requires set up.

    In 1971, Buick went to the 8.5 10 bolt center section. Lots of parts availability for that rear. If you can find an 8.5 rear from a 1971-72 Skylark or Cutlass, it will bolt right in, just have to shorten the drive shaft.
     
  13. Jerry68GS400

    Jerry68GS400 No Daddy, my GS

    I see.... I saw several listings on eBay mentioning posi/gear sets for 8.2. I did not read any further so those were probably for the 64-67.
    And thanks for the info on the transmission. I was, of course like everyone else, trying to find a way to get 25mpg while melting my tires at every stop light.

    Of course I realize there is more to setting, shimming up the differential, but I can assume that if I find, which I have, a 71-72 open wheel rear, I can use that housing to purchase a posi unit and install without having to modify the bearing bosses?

    Sorry if I am sounding ridiculous asking these questions.
     
  14. LARRY70GS

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

    Yup, we get lots of questions wondering why the 8.2 gears or posi won't fit in their 68-70 Buick 8.2 rear. Everyone wants their cake and eat it too. It certainly is possible to have a tire burner that sips gas, but that is not so easy to do with a lot of cubic inches. Better to start with a smaller engine and add boost for on demand power. Couple that with electronic EFI and an OD transmission with performance ratio rear, and you can get there. How deep is your wallet?:)
     
  15. alvareracing

    alvareracing Platinum Level Contributor

    I have a 1971 10 bolt open with 3:23's for sale if you need one. PM me, I can bring it to the Nationals if you like.
    Fernando
     
  16. Jerry68GS400

    Jerry68GS400 No Daddy, my GS

    Like your avatar Fernando.

    Your post leads me to another dumb question. It probably would not make any sense to, but could you take those 3:23s ring off the open carrier and directly install onto a posi carrier?

    And thanks for the offer, but I have a 8.5 in a parts Skylark behind the house. So if anyone else is interested in his 8.5, they can contact you. Do you have a asking price?
    And I am so out of the loop, not sure where the Nationals are. I have definitely heard of the Nationals.
     
  17. LARRY70GS

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

  18. Jerry68GS400

    Jerry68GS400 No Daddy, my GS

    Yes. I knew that. ;)
     
  19. BQUICK

    BQUICK Gold Level Contributor

    I found the 2.56 only got better mileage at 75-80 mph. Below that the converter slippage made it inefficient. Lockup converter would be the ticket. But then you will have speeding tickets.....I went from 2.56 back to 3.90...now I might get a ticket for going too slow....at least I don't run in the left lane. Have to deal with Priuses tailgating me. One time this woman was right on me wouldn't go around. At red light I sat and she honked...then left her in a cloud of tire smoke. I need to grow up....or maybe not....
     
  20. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    You could easily install a posi carrier into the open 3.23 rear. You would not have to mess with the pinion depth. You wouldn't even have to remove the pinion. Measure the backlash before you remove the open carrier and then install the posi carrier. Set the backlash so it's the same and then go racing...
     

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