Check my math

Discussion in '1965 - 1973 Classic Mustangs -General/Talk-' started by Rmoore45, Dec 8, 2012.

  1. Ok I'm fairly certain this adds up but I just want to be sure.

    I have a T-5 with a 3.35 first and .68 OD (according to the tag on the trans). My rear end gears are currently 2.79

    My overall gear ratio in first gear is (2.79x3.35) 9.35
    My overall gear ratio in fifth gear is (2.79x.68) 1.9

    Reason I'm asking is because I'm thinking of getting 3.55 gears and if I multiply .68x3.55 I get 2.41 as a final gear. Which is still lower that my original 2.79, so if I go with that gear my rpms will be lower in fifth gear than if I was in 4th gear with my 2.79s.

    I would just like to be sure that my math is correct before I start buying things.

  2. It looks correct to me.

    That's the reason we like overdrive transmissions. ;)
  3. Yeah, Yeah, Yeah :rolleyes:

    I'm just making sure that I multiply the correct numbers so I don't screw myself if I get the 3.55s
  4. I don't think you'll be disappointed with them. You'll get a noticeable seat of the pants improvement and still keep it under 1800rpm at cruise. I have 3.50s and no OD...does about 3000rpm at 65mph. Why are my ears ringing? o_O
  5. Hmmm....I always thought the numbers were added not multiplied..
  6. Nope, it's multiplication and division.
  7. If you want to get more accurate with RPM's at any speed figure up your tire ht. then plug it into the RPM formula here with different rear gear choices. I made a spreadsheet to get an overview of what each rear gear change would do to hwy speeds to pick the best gears for my use.
  8. There are a lot of steps involved, but the math to do it manually isn't very rigorous. I have 235/60-R15s on the back, 3.50 gears, and a 1.00 fourth rati0, so:

    235mm × 0.60 = 141mm sidewall height.
    141 × 2 = 282mm ; the amount of total tire height due to the sidewalls.
    282 ÷ 25.4mm/in = 11.1"
    11.1 + 15 rim = 26.1" overall tire height.
    26.1 × π = 82.0" tire circumference. (If using a standard calculator, the constant π (pi) = 3.14159.)
    82.0 ÷ 63,360 inches per mile = tire circumference of 0.00129 mile.
    55mph ÷ 0.00129 = 42,496 rear wheel revolutions per hour.
    42,496 ÷ 60min/hr = 708rpm at the rear wheel at 55mph.
    708 × 3.50 gear ratio = 2,480 engine rpm at 55mph.

    Again, it's a lot of steps, but it only takes maybe 30s to punch it all into a calculator. It could also be entered into a program like Maple or Microsoft Math as one big expression and repeated quickly with different variables.......or you could use a spreadsheet.
  9. Is there a way to figure out what the gear ratio is in my c-4 trans? I had aamco rebuild it for me.They put in all b&m clutches in it. I have an 8" open end rear w/4.11 in it. It leaves two tire marks in 1st and 2nd but just one tire mark in 3rd. I need an od in mine cause it revs too high at highway speeds.I have the motor pulled out right now cause the body needs a resto bad.I know how to test the rear for gear ratio but can you do it for the trans by puttin it in 1st and spinnin the tire??
  10. The gear ratios for a C-4 are
    1st: 2.46
    2nd: 1.46
    3rd: 1
    Reverse: 2.18

    So with 4.11 gears
    2.46 x 4.11= 10.11
    1.46 x 4.11= 6
    1 x 4.11= 4.11
    2.18 x 4.11= 8.96

    These are the exact gear ratios, since the C-4 doesn't have a lockup converter they will be a little bit off since the torque converter is slipping a little bit.

    You're probably doing about 3600 rpm @ 65mph aren't you?
  11. I don't recall what my rpm was at 65 but to sit at a steady speed like that was uncomfortable because the engine was screamin. A definite upgrade will be an aftermarket aod.I'll have to do some figgerin to see if i want to stay with 4.11 or go with somethin a lil more highway friendly. I jus love that neck snappin response with the 4.11. Thanks for the c-4 ratios...much appreciated...and Thanks Everyone for the math thread and your inputs.
  12. my fox with the same t5 you have, a 25.6" tire (245/50/16) and 3.73s used to do 2300 rpm at 70mph. so you would be a hundred or two rpm lower at that speed with 3.55s and a tire around that diameter.

    the formula is

    [(Tire Radius * rpm)/168]/drive ratio

    (3.55 * .68 in your case)

    as an example

    13 (for a true 26" tire) x 2000 rpm = 26000

    26000/168 = 154.76

    154.76/ 2.41 (3.55 x .68) = 64.2 mph

    this does not take converter slippage into account.
  13. Fow what its worth, I have a T5 and 3.55 gears and think is perfect for a street car.