From 2.56 to 3.42 gears, mpg/rpm difference?

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

  1. buicks

    buicks Well-Known Member

    Anybody done this swap? Or maybe a 3.23 swap?
    I have a 350 eng and TH350 trans, 72 skylark, stock engine with headers.
  2. Tim N.

    Tim N. Platinum Level Contributor

  3. Briz

    Briz Platinum Level Contributor

    Between switching from the 3:07 to the 3:42 and dropping in 100 more HP my milage went from 15-18 city/Hwy to around 7-10 mpg I was running around 2600RPM @ 70MPH to 3000@ RPM. Ive gotten used to thinking If you own a big block you dont really car about fuel milage.
    sriley531 likes this.
  4. Ryans-GSX

    Ryans-GSX Have fun, life is short.

    I have had many different gears in cars and I personally like the 3.23 or 3.08 best for a all around street gear the 3.42 is a bit hard on our cars. The highway speed limit of 70 mph will cause you to tach high and our cars where made with a 55 mph limit in mind.

    I like to drive my car on the highway so this is a factor for me.
    angelorayhurst likes this.
  5. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    336/tire height X gear ratio X MPH = RPM with no converter slip. ADD 150-200 rpm for converter slip. Stock tire height is 26.6"

    336/26.6 X 2.56 X 60 = 1940 RPM

    336/26.6 X 3.42 X 60 = 2592 RPM
  6. John Codman

    John Codman Platinum Level Contributor

    Assuming a 1:1 top gear, I agree with you. A torque monster engine such as the bigger Nailheads will go well with a 2:93. If you are interested in drag racing, you obviously need a lower final gear. A 3:42 is great with an overdrive such as a 700R4. I always gear my cars for the highway, so personally with a non-overdrive tranny, I would pick a 2:73. I hate seeing 3,000 RPM on the highway.
  7. elagache

    elagache Platinum Level Contributor

    Gotta choose . . .(Re: From 2.56 to 3.42 gears, mpg/rpm difference?)

    Dear Jason and V-8 Buick comparison shoppers . . . . .

    Larry has given you the RPM difference and Briz has given you an idea of the cost you'll pay in terms of gas mileage. There are no miracles here. If you ask your engine to turn 33% more per minute, the engine will burn more gas in order to do that. When cruising on the highway, you don't need that additional power generated at the higher RPM, so that fuel is basically being wasted.

    That's the reason why an overdrive transmission is the single most important investment you can make if you want better gas mileage.

    Cheers, Edouard :beer
  8. mikesstage

    mikesstage Guest

    nice to be retired Im 12/31
  9. buicks

    buicks Well-Known Member

    How about a 350? Without internal perf parts. (stock cam)

    ---------- Post added at 01:14 PM ---------- Previous post was at 01:10 PM ----------

    Nice thanks.

    336/26.6 X 3.08 X 60 = 2334 RPM

    336/26.6 X 3.23 X 60 = 2448 RPM

    (wish I had a tach in my Skylark) :)
  10. gscalifornia

    gscalifornia Small blocks rule!!

    If you're worried about gas mileage and still want to be able to cruise at 70 I wouldn't go any higher than a 3.23 gear if you're staying with the T350 trans.

    That's the setup I have in my '69 and with the stock 26" tall rear tire I'm pushing about 3300rpms at 70mph. When I had the 2.73 rear gear I could run 70mph at about 2900rpm and get about 18-19mpg, now with the 3.23 I get about 13-14mpg because of the higher rpms.

    It also gets a little unnerving running for long distances at that high an rpm.
  11. 69GS400s

    69GS400s own amusement ride!

    Re: Gotta choose . . .(Re: From 2.56 to 3.42 gears, mpg/rpm difference?)

    Actually its just a change of measuring units from Miles Per Gallon to Smiles Per Gallon :TU:

    ... but I do find your before / after #'s to be quite severe :Do No: I went from 3.08's to 3.31's and saw at most 2 mpg difference running @ 70-75mph. Right around 2900-3000 rpms I can feel the cam coming on and the motor take on a different tone.

    I dont think that 55 was the speed limit in 68-72 in most places

    Larry has pretty well proven that an OD isn't going to release the magic mileage genie from his gas tank with a high hp motor
  12. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Figure on 2-4 MPG depending on how you drive. And there is no reason you can't have a tachometer in your Skylark.

    ---------- Post added at 02:43 PM ---------- Previous post was at 02:35 PM ----------

    That is true. The best I have seen with my old engine was 14-15 MPG, 16 once on gasoline that didn't have any ethanol. I think I can equal that with this new motor, but I will need to lean out the carburetor. I plan on putting a Q-jet that I got from Everyday Performance for longer trips. It has an Adjustable Part Throttle on it. Of course the best solution is Fuel Injection. All my MPG calculations have been with a jetted Q-jet, and the AED 1000. The best I ever did with the AED was 13.9 MPG.

    The one nice thing about overdrives is the reduction in RPM on the highway.
  13. Joe B

    Joe B Well-Known Member

    Actually, these cars were built for enthusiastic driving, street racing, and performance, rather than economy. The 3:23 and, to a lesser extent, 3:42 ratios were something of a compromise for drivability. The cars were not built for a 55 mph speed limit. The speed limit for most of the country at the time these cars were built was 70 mph. The 55 mph speed limit was not implemented until the 1973 gas shortages. I really don't think that if the auto industry had anticipated the coming gas shortage they would have built these cars with 3:42 or higher rear ends, thirsty big block engines, or such heavy bodies. At the time, most folks with these cars didn't care about high rpm at cruising speeds.
    BeatersRus likes this.
  14. elagache

    elagache Platinum Level Contributor

    Touch - still a tradeoff (Re: mpg/rpm difference?)

    Dear Mike, Jason, Ken, Alan, Larry, Joe, and V-8 Buick "cat skinners" . . . .

    Actually, this is a big part of deciding how to invest in your classic. If you look at the total cost of ownership, you need to spend an awful lot of gas before improvements in gas mileage really add up to a significant cost savings. If the car is just a weekend cruiser and you never go very far, the answer is to just hold your nose and pay what it takes to fill up the tank. You'll probably drive the car less when gas is especially expensive.

    It does take a certain fanaticism to overcome the resistance to pay so much for fuel just to have fun. Certainly some number of fellows give up on their cars each year and the high price of fueling is just one of the annoyances that finally make them give up.

    Yes Alan this is true. However, if you going try to improve your gas mileage in a piecemeal fashion, the overdrive tranny will have the most noticeable benefit. It is hard to get even 2-4 MPG improvements with any other single change (assuming you keep the same engine.) It is equally true that it may not be the best return on your investment given the relatively high cost of a new transmission (compared to the other tweaks you might try to do to improve gas mileage.) What makes the overdrive transmission appealing is that it doesn't impede performance like so many other techniques to improve gas mileage.

    If you really want to improve your gas mileage by leaps and bounds . . . Yeah, you need a systemic approach and must be prepared to change lots of things. We'll see if all these theories actually work once my wagon finally gets back on the road!!

    Cheers, Edouard :beer
  15. Mark Demko

    Mark Demko Well-Known Member

    Since your 350 is stock, I'd go no higher than 3.08, no benefit to wingin' the engine with a stock cam. I went from 2.41 to 3.42 in an '82 Regal with the 3.8 V6 with a 350 trans, all I gained was RPM really. It had a mild KB cam, KB intake, Holley 390 4bbl.
    Last edited: Oct 12, 2012
  16. John Codman

    John Codman Platinum Level Contributor

    Re: Touch - still a tradeoff (Re: mpg/rpm difference?)

    Actually, in my '79 Corvette, by changing the final drive ratio from 3:78 to 3:08 the fuel economy went from 11-12 mpg to about 17 mpg on the highway.
  17. yuk

    yuk Well-Known Member

    here are some of my roadmaster wagon results.

    my 2.73 gear with the tires i use is almost exactly the same final ratio as a 2.56 with 225 75 15 tires.
    PM me an email addy with a "geardaddy" request and i will send you a cool spreadsheet that i made many years ago that help calc out final drive ratios, concrete, some hyraulic fluid capacities and few other fun things.
  18. Jerry68GS400

    Jerry68GS400 No Daddy, my GS

    I figured I would rekindle this post. I picked up an Olds posi 12bolt rear cover, 10 bolt carrier pretty cheap with 2.56 gear ratio. My GS 400 lost its original posi rear end decades ago and has been running with a stock open wheel Skylark differential. I have a few questions like:
    1) is that Olds O differential worth investing in to rebuild?
    2) how much gas mileage will I really achieve with that gear ratio swap? I can’t remember how much gas was back in 2012, but see it is on the rise.
    3) will that Olds O swap right in to the 1968 Buick or are there mods that are needed.
  19. TrunkMonkey

    TrunkMonkey Well-Known Member

    Another thing to consider about the RPM at cruise. (broad brushing)

    The focus as a balance of efficient operation. Best MPG at cruise.
    Coupled with the best performance. Best HP/Torque curve at cruise.
    And smoothest point of RPM.

    All things were to meet that "happy spot". So, some cars had poorer offline acceleration, some had less than smooth idle, or poorer economy in stop and go or slow speed driving.

    But were optimal in their most expected operating range.

    A lot of these cars would "lug" below 2000 RPM, and you almost never gave full throttle in 1:1 below 2-3000 RPM, either manually downshift, of automatic kick down.

    It was in the 80s that overdrive and computer managed systems, coupled with added transmission gears etc, gave us cars that run well at 1500-1800 on the highway, and 2000 RPM in moderate driving around town.

    Running 2500-3000 at highway for sustained time was the norm.

    Most "driver/cruser" cars are fine. Those pushing higher performance builds need to optimize and target the expected plave to run.

    And with the wealth of accumulated information than what we had 40+ years ago is better then the old way of sticking some lumpy cam, slapping on a "performance intake" or any big carb, or a set of headers in an engine with less than optimal gearing, or an engine torque/HP range that was outside the transmission gearing (manual) and converter stall and shift points for an auto.

    And then scratching our heads while looking at an empty wallet and a dog of a car.

    But, that is part of the journey we took to get here.

    Do the math, ask questions. So many have already "done the heavy work", honor their efforts, and apply their knowledge and be generous and honest for it.

    And have fun, make it infectious. Do it for the chirrenz!

    Harlockssx likes this.
  20. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    The Olds 12 bolt isn’t a bad rear, but I believe it may be more expensive to build as far as parts availability is concerned. The best rear remains the 71-2 Buick or Olds 8.5 10 bolt. Lots of parts available for them and they are bolt in axle. The 64-67 BOP 8.2 10 bolt rears are also bolt in axle and parts are plentiful. The 68-70 Buick 8.2 is the rear to avoid as parts for them are more expensive and hard to find although that has improved over the years. The rears bolt right in. You may need to shorten your drive shaft.

    The 2.56 ratio will give you roughly 30 MPH/1000 RPM. Gas mileage will be very good at the expense of some performance.

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