Rear End Gear Ratio--39 Ford

[jon laing]

How do I determine the rear end gear ratio on my stock 1939 deluxe ford 2 door sedan?

[G.M.]

You can mark the crankshaft and mark a tire at the floor. Put the transmission in high gear and push the car by hand. One complete turn of the wheel and watch the crank shaft counting the number of turns. A little over 4 turns= 4:11, 3 3/4=3:78,
3 1/2=3:54. In my opinion from several hundred thousand mile on old Fords over the past 30 years mostly in 39's the best rear is a 3:78 with a Columbia 2 speed which is a 28% over drive. At 65 MPH the engine sounds like 45. My second choice would be a 3:54. G.M.

[jon laing]

Thanks for reply. I will do your test. Seems like they would have stamped this info someplace on the car. Jon Laing

[Pat/Ohio]

Look on the bottom of the pinion housing....most are stamped (i.e. 9 37 = 4.11 etc.). But sometimes these gears get changed over the years.

[JM 35 Sedan]

There should be numbers stamped on the bottom of the banjo housing on the center web near where the torque tube bolts up. Divide the larger number by the smaller number will give you a final gear ratio. Example: 39 ÷ 11 = 3.54:1 ratio, 37 ÷ 9 = 4.11:1 ratio, and 34 ÷ 9 = 3.78:1 ratio. However, these stamped numbers on the banjo may no longer be correct if someone has previously changed the original gears. A couple other methods are:

1. Drain the oil by removing bottom pipe plug. Remove rear pipe plug and wipe one tooth clean on the ring gear. Mark that tooth with a felt tip pen. Count the number of teeth on the ring gear and that number should match one of the three ratios above.

2. Jack up the left rear tire enough to clear the ground. Mark a line on the front face of the tire with white chalk. Put the transmission in third/ high gear. Turn the engine two revolutions by using a socket on the crankshaft pulley hex bolt and note how much the tire turns based on the chalk line. I will come back and add a link here that gives more details on this method.

http://www.btc-bci.com/~billben/reargear.htm

[jon laing]

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OK---How do Identify the crankshaft pulley hex bolt? Do I turn it clockwise or counter clockwise? Sorry for such simple questions. Jon Laing

[OLD...BILL]

Originally Posted by G.M.

You can mark the crankshaft and mark a tire at the floor. Put the transmission in high gear and push the car by hand. One complete turn of the wheel and watch the crank shaft counting the number of turns. A little over 4 turns= 4:11, 3 3/4=3:78,
3 1/2=3:54. In my opinion from several hundred thousand mile on old Fords over the past 30 years mostly in 39's the best rear is a 3:78 with a Columbia 2 speed which is a 28% over drive. At 65 MPH the engine sounds like 45. My second choice would be a 3:54. G.M.

Thats what my 39 fordor has ...3.78s with Columbia ..... OLD....BILL

[VeryTangled]

Hello Jon Laing, the bolt you're asking about is the one sticking out the front end of the crankshaft. It's where the crank would go if you were hand-starting it. It should be just below the distributor. Facing the front of the car, turn it clockwise.

-VT/JeffH

[cederholm]

Resurfacing a very old thread, can someone explain this on what I believe is a ‘39 transmission.

Thank you.

[Ggmac]

That looks like your differential, (banjo rearend ) and it looks like a 4.4 rear ratio. Was yours a 60 hp v8 . I’m sure someone with more knowledge and a better memory than me will chime in .

[V8COOPMAN]

Originally Posted by Ggmac

That looks like your differential, (banjo rearend ) and it looks like a 4.4 rear ratio. Was yours a 60 hp v8 . I’m sure someone with more knowledge and a better memory than me will chime in .

You seem to be correct .... a 4.44 rear gear was common for the V8-60 engine.


Coop


.

[cederholm]

Thank you guys. I will be using this behind a 52 8BA that has been mildly hopped up.

How did you decipher it’s 4.44 from those numbers?

[cadillac512]

Divide 40 (teeth on the ring gear) by 9 (teeth on the pinion) and you get 4.44

[blucar]

Originally Posted by cederholm

Thank you guys. I will be using this behind a 52 8BA that has been mildly hopped up.

How did you decipher it’s 4.44 from those numbers?

The 4.44 ratio behind an 8BA engine would be OK for 1/4 mile drag racing, a 3.78 would be better for every day driving..
My 36 coupe has 3.78 with a Columbia, 59 AB engine that has been modified to a high level. Car runs very well, street or hiway.

[cederholm]

AH HA! That’s a 4 (40), I thought it was a 1 (10) which made no sense at all.

Thank you.

Originally Posted by cadillac512

Divide 40 (teeth on the ring gear) by 9 (teeth on the pinion) and you get 4.44

[Ggmac]

You should take your tire height and if bias or radial into consideration. A shorter (radial ) tire would give your 4.4 ratio an even more radical ratio , say 4.56 or more .
There is a formula for tire height and increase / decrease in actual ratio . You don’t want to have that motor screaming in high gear and only doing 55 .

[blucar]

The tires on my '36 coupe are 600x16 which are a 28" tire, approximately the same size as a 235/75R 15.

[Ford Freak]

My '37 coupe with a stock '39 flatty (85 hp) ran great w/3:54's.