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Technical Flatheads and gear ratios

Discussion in 'Traditional Hot Rods' started by TCATTC, Oct 27, 2020.

    Joined: Oct 12, 2019
    Posts: 244


    I figure some of you Flathead Ford experts can give me some gear tips. I have a 8RT that came out of my '53 F-100 that I am about to build a Model A pickup with. The F-100 had 3.90 gears. I was wondering what kind of gear ratio a near stock flattie/T-5 combo would like in a light Model A. I know this has probably been discussed before but I can't find anything on it.
  2. oldsman41
    Joined: Jun 25, 2010
    Posts: 1,282


    My 51merc weighs twice what your truck will I run the 3 spd od trans and 4 11 gear you could have a real hot rod with that gear
  3. flatheadpete
    Joined: Oct 29, 2003
    Posts: 10,112

    from Burton, MI

    My T-bucket with a built '51 Merc and a T5 had a 3.78 banjo rearend. I was decent on gas and accelerated rather well with the steep 1st gear.
  4. sloppy jalopies
    Joined: Jun 29, 2015
    Posts: 4,586

    sloppy jalopies

    tranny shop owner told me that my 351 / AOD ford liked 3.70 and steeper... 7.60 - 15"...
    memory says 1800 RPM @ 65 MPH...
    about 450 RPMs difference between drive and overdrive...

  5. I'm running a stock 1940 221ci flatty in my beater panel with 3.0 gears out back. My 4 speed that I have behind it is the magic to that equation. Saginaw 3 grooves: First 3.50, Second 2.46, Third 1.65, Fourth, 1.00

    It does fantastic around town in 1-3. Getting on the highway, I run it up to 55, or so, then shift into 4th. It really likes to run down the highway around 70-75.
    R A Wrench and studebaker46 like this.
  6. Norris McCarty
    Joined: Apr 19, 2007
    Posts: 322

    Norris McCarty

    40 Coupe with 53 Merc, T5 and 3.80 gears. 29” rear tires.
    Last edited: Oct 27, 2020
  7. Bandit Billy
    Joined: Sep 16, 2014
    Posts: 7,515

    Bandit Billy

    Not all T-5's have the same 1st gear so it would be a guessing game to tell you the rear ratio.
    flatheadpete likes this.
  8. mkubacak
    Joined: Jun 20, 2005
    Posts: 183


    Don't forget that tire diameter is a consideration also.

    I did not like the idea of a 4.03 first gear from the S-10 T5 gearset, so I am building a franken-T5 with a S-10 4x4 case to keep the closed driveshaft with an adapter from Vintage Metalworks, and the gears from a NWC Camaro T5 to get the 2.95 first gear. I will be using this close ratio box with 3.78 rear gear and 31" tall tires.

    Punch your numbers in here and you can compare different results.
    47chevycoupe and Bandit Billy like this.
  9. Bandit Billy
    Joined: Sep 16, 2014
    Posts: 7,515

    Bandit Billy

    8BA with a NWC with 3.75 first and a .72 OD. Tires are 30" tall and the rear is a Winters Champ QC with 4:11.
  10. fh4ever
    Joined: Apr 30, 2019
    Posts: 5


    51 F1 with 8BA with T5 (S10 gears) and 3.92 rear on 255/60R15. First gear is too low, I take off in second often, and comfy at 70-75mph interstate speeds
  11. The '35 Fordor had a t-5 and 3:55 8 rear. The wooki (vicki length woody) that I'm building will have a modified 8BA, 5 spd and 3.78 banjo.
    Jason Mitchell likes this.
  12. tjm73
    Joined: Feb 17, 2006
    Posts: 3,386


    Stock V8 flatty 3 speed gear ratios are 2.82/1.60/1.00 from what I read with a 3.78 R&P. So a T5 with a 2.95 geared T5 would be a very good match IMO. Couple that with a 0.80 overdrive and that cut the high revs like you're running roughly a 3.00 R&P.
  13. Pete1
    Joined: Aug 23, 2004
    Posts: 1,917

    from Wa.

    It depends a lot on your driving style and what you want the car to do.

    Do you like to hear the engine performing at best all the time?

    Do you lug around town most of the time?

    Do you mind shifting so the engine doesn’t bog?

    Do you do a lot of “cruising” the avenue?

    Do you do parades?

    Are you going to race at Bonneville?

    The ratios you need in the transmission and rear end are ALL dependent on these questions.

    Setting up for one parameter will compromise others.

    One setup is not a “fitz all”.

    There are more T5 gear sets than you can count on 2 hands.

    A 4.11 rear gear will be best for a stock engine. That could give you a final overdrive ratio that your stock engine could pull on the freeway ok if you have a .82 or .86 overdrive gear. The rest of the gear set is up to you.
    Bandit Billy and Jason Mitchell like this.
  14. hemihotrod66
    Joined: May 5, 2019
    Posts: 311


    A stock 1934 Ford came from the factory with a 4.11 gear or optional 3.55..
  15. Garpo
    Joined: Jul 16, 2016
    Posts: 208


    Its all covered in basic high school maths.
    Measure tire diameter in inches and multiply by 3.14 to get circumference
    Divide that into 63360 for tire (wheel) revs per mile.
    Multiply by rear end ratio for direct top engine revs per mile (or rpm at 60mph.)
    From there it is simple to calculate engine revs at your targeted highway cruise speed.
    Ditto calculate speed in each gear
    Playing around with the numbers is far cheaper than guessing and getting the answer wrong
    You do need to be honest about how you intend to drive it, but the numbers don't lie.
    bchrismer likes this.
  16. Jason Mitchell
    Joined: Sep 4, 2020
    Posts: 40

    Jason Mitchell

    Cool. I'm doing 8BA with 3.78 banjo, but '39 Trans. BTW...I got a tattoo in Ellicott City.
  17. Boryca
    Joined: Jul 18, 2011
    Posts: 695

    from Detroit

    I'm running a lightly built 59AB, '39 3-speed (3.11 - 1st; 1.77 - 2nd; 1:1 3rd) with Columbia rear on 3.0:1 R&P, and do a lot of mixed-use driving in my '35 pickup (light).

    In town: drives great, no issues, but there's no stoplight racing.
    Highway: runs like a bat out'a hell - 85+ all day if you've got the cajones to do it on a stock banjo rear, but you'll get passed a few times while accelerating for sure.
  18. TCATTC
    Joined: Oct 12, 2019
    Posts: 244


    Thanks everybody
  19. The 39 guy
    Joined: Nov 5, 2010
    Posts: 2,985

    The 39 guy

    I just finished installing a WC T5 5 speed in my 40 Ford Coupe. I used the calculator to make the chart you see below. You just fill in the numbers and it calculates your rpm at each speed. I found this chart to be right on with the RPM and speeds I actually run in my car.

    Your truck and my car should be pretty close on weight and horsepower. I have some speed goody's in my flatty but I don't think I gained all that much power with the mods. I am very happy with the drive-ability of the car with the 5 spreed and 3.50 gear. I was not happy with the first gear and 3.89 gear combination. I like to keep my RPM's down below 3000 on my engine. From what I have read the flathead engines make the most power and torque below 2000 RPM. I find that the car cruises well at 1800 rpm at 60 mph and can easily pick up speed in 5th gear to pass someone and it climbs hills well in 5th while 4th gear is in reserve for the steepest hills.

    As others have said what you end up using ( what the truck needs) depends on several factors so consider them all before spending money on the parts.

    I decided to leave the 3.89 gear in my car after switching from the 3 speed overdrive I had in the car previously but decided to experiment with the 3.50 to get better driveability and I am glad I did. I will be keeping the 3.50 gear in the car because it makes me smile! You can see more details about the transmission and the rear end swap on the last page or two of the Uncle Mike Build thread in my signature line or what ever they call that.

    3.89 3.50 Chart JPG.JPG

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