Gearing or new tranny?

Discussion in '1955-1959' started by garrison, Oct 5, 2019.

  1. garrison

    garrison Member

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    I have a 57 Chevy truck. It's stock except that the gentleman I bought it from told me he put different gears in the rear. But it tops out at 55-60? Do I need new gears or a new transmission to hit 70mph? My uncle gave me a 700R4 tranny, but I don't want to lose the column shift just to have an automatic UNLESS it is necessary to get my highway speed. It seems everywhere I look I get different answers. Soooo....:
    A) If I can just regear... what gearing do I use and where do you recommend getting it?
    B)If I need the transmission switch, can I keep the column shift? Do I still need to regear in addition to the tranny?
     
  2. Bill Hanlon

    Bill Hanlon Member

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    First, use a GPS app (your phone probably has one) to verify that you are actually going 60 when the speedometer says you are.

    Next, find out what ratio gears are in the back. Make a chalk mark on the driveshaft and another at the bottom of a rear tire. Lay down beside the truck and have a friend push the truck exactly 1 revolution of the rear tire while you count evolutions of the driveshaft. Once you've moved the truck exactly 1 rev of the rear tire, estimate how far into the last rev the driveshaft has turned. If for instance the driveshaft has turned 4 complete revs and a little more, you probably have a 4.11 to 1 rear gear set. Just under 4 turns is likely a 3.90 to 1 gearset. 3 and a half revs is likely a 3.55 to 1 gearset. What did you find?

    Now measure the diameter of the rear tire. It will likely be in the 25-30 inch range. What is it?

    The 700R4 would reduce engine speed by 30% in overdrive vs. high gear on your 3 speed. IF (big if) your engine is running well enough to get back to the same RPM you are currently running at 60 MPH, you should be able to reach around 84. My guess is that the engine doesn't have that much UMPH, and you'd probably back out of the throttle before that anyway unless your front end was in real good shape.

    If you do decide on installing the 700R4 you will need an adapter like this: Store - Langdon's Stovebolt and probably should use their starter (another $60).
     
  3. garrison

    garrison Member

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    Thanks!!! I had watched a youtube video () on determining ratio. Here's what I did: jacked up the rear end and blocked on tire. Turned the wheel 10 times exactly and counted driveshaft revolutions. It turned 16.75 times. According to the video instructions it came out to about a 3.35? Does that make sense? I am truly ignorant on this subject. But I'll follow your suggestions this weekend and recheck.
     
  4. Bill Hanlon

    Bill Hanlon Member

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    That youtube video's method is also valid.

    Probably 3.38, which I don't think was originally a truck option from Chevy. Our host (among others) does sell a 3.38 gearset to upgrade the stock rear end for higher speed operation.

    Let's assume that you do have a 3.38 to 1 ratio and that your rear tires are 26 inches tall. At 60 MPH your six cylinder engine is turning 2623 RPM. The '57 six cylinder engine had a horsepower peak of 140hp at 4200 rpm, which with your current gearset and transmission would be 96 mph.

    BUT (big but) your engine only has enough power to get you to 60 mph. And if you put in the 700R4, your truck should still get to 60 mph in 3rd gear and would actually slow down when it shifted into 4th (overdrive) gear because it doesn't have enough power to overcome the additional load in OD.

    I think you need to get your engine running well and forget the OD tranny.

    By the way, you would have received a lot more input on this thread if you had posted it in the "Chevy & GMC Trucks by Year" "1955-1959" category instead of the "Classic Parts of America" "Classic Parts of America" category. I'll see if I can get Nick (the forum moderator) to move it there.
     
  5. garrison

    garrison Member

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    THANKS! I really appreciate it!
     
  6. dorcutt

    dorcutt Member

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    I have a similar situation on my 57 as the author of this thread has but have a Th 350 tranny already installed. I emailed Tom and his recommendation was to go to a 700 tranny verses changing the gear ratio to 3.38. As this is my summer ride I’m still debating on whether I want to put that kind of money into it or not.
    I did jack the truck up and marked both the tire and drive shaft and for every rotation of the tire I had a full rotation of the drive shaft so I’m doing something wrong I suspect. I will try the method in the video.
     
  7. coilover

    coilover Member

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    Your original post does not say that it has a six in it. If it's a V8 then the 700R4 will bolt up to it. Your three speed column shifter will work with any automatic ever made be it 2sp Powerglide, 3sp Turbo 350, or 4sp (OD) 2004R or 700R4. The shifter is dumb and doesn't know whether it's moving gear levers or auto selector lever. Pic of a factory column and shifter in a 53 that also has a start in park only safety switch. An OD automatic will give a final drive ratio of 2.88 with a 4.11 rear and 2.73 with a 3.90---both would just loaf at 70mph. A search should show how to keep the column shift or I can send you a step by step.

    S10 055.jpg
     
  8. Bill Hanlon

    Bill Hanlon Member

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    I'm pretty sure he has a six cylinder engine Evan. The picture in his avatar doesn't have a V on the side of the truck and he didn't correct me when I said:
    " Let's assume that you do have a 3.38 to 1 ratio and that your rear tires are 26 inches tall. At 60 MPH your six cylinder engine is turning 2623 RPM. The '57 six cylinder engine had a horsepower peak of 140hp at 4200 rpm, which with your current gearset and transmission would be 96 mph."

    But I still stand by my statement that if the 1:1 ratio high gear he currently has with the 3.38:1 rear gear he currently has can only push the truck to 60 MPH it doesn't have enough power to get past 2623 RPM in high gear with the current final drive ratio. Putting an OD in isn't going to help that situation. My guess is that it would actually reduce max speed in OD below the 60 MPH he can now reach. He would be able to reach 60 in 3rd gear (1:1) in the 200R4/700R4.

    He needs to get his engine running well first.
     
  9. dorcutt

    dorcutt Member

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    Guys I found a formula on an earlier thread to help me understand what my RPM would be with different gear ratios. If I understand it correctly my RPM should drop almost 500 RPM if I go from 4.11 gears to 3.38 with a 28 inch tire. Is my thinking correct.
     
  10. Bill Hanlon

    Bill Hanlon Member

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    You left out one variable. At what speed?
     
  11. dorcutt

    dorcutt Member

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    Thanks Bill, I would like to cruise comfortably at 60/65. At 50 right now it she seems pretty wound up. I still have to confirm my rear end gearing as I suspect it’s original at 4.11.
    My 51 has the patricks. 3.55 gearing with the same engine and cruises pretty good at 55.
     
  12. Bill Hanlon

    Bill Hanlon Member

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    With a 28" tire, RPM would drop from 3004 to 2596 at 60 MPH if you changed 4.11 gears to 3.55.
     
  13. dorcutt

    dorcutt Member

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    Many thanks for responding Bill, happy thanksgiving. I hav another question for you but will open up a new thread.
     

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