What gear ratio do I have?

C

Curly

Member
I was reading a thread earlier today on figuring out gearing ratios. I can't seem to find it now. Forgive me if this is a re-post.

I lifted one rear wheel and cranked the handle exactly 15 revolutions. The lifted rear wheel made 8 full revolutions. The truck was in 3rd gear. This is a 1939 transmission.
Can anyone tell me what gears I have in the rear end?
 
Thanks Tom. 2250 RPM's = 48 mph. 16 inch wheels. Might be time to look into some higher speed gearing from Snyders. Is there a chart or formula to see how many mph I might gain? I wouldn't mind gaining 5 mph.
 
Curly said:
Thanks Tom. 2250 RPM's = 48 mph. 16 inch wheels. Might be time to look into some higher speed gearing from Snyders. Is there a chart or formula to see how many mph I might gain? I wouldn't mind gaining 5 mph.
Click to expand...

Larger tyres will give you a little more speed and more rubber on the road.
I gain 5% by using 5.25X21 tyres instead of the original 4.5X21 with better road holding and no adverse affects.
 
3:78 is a good all around gear. Why spend time & money for just 5 MPH gain????---"Maybe" your speedo's not even accurate, anyhow????
Bill Cheep
 
Help me with the math here please- 8 divided by 15 is 1.875, not 3.78. Do you have to double the 1.875 because one wheel was left on the ground, causing the differential action, and the spider gears have half the number of teeth than the axle gear?
 
Mr. Cheep,

5 mph gain is pretty good when on club rides you are always about 5 mph slow. By the time I get to the pie stop all the good pie is gone.

:)
 
The attached may help answer the original question. Document provided by MAFCA.

Tom Endy
 

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A 3:54 should give about 4 mph at same engine speed over a 3:78
A 3:27 would be about an 8 mph increase over the 3:78
 
Wilk N; Take the 8 and knock it in half since only one wheel is turning. Then divide the 4 into 15 and you should get close to 3.78. 3.75 to be exact but the standard ratio was 3.78 so that's what he has. Jack
 
Thanks Jack- and i suspect that the reason you knock the 8 in half is due to the number of teeth on the axle gear vs. spider gear. When both wheels are turning the same speed, there is no relative movement between the spider and axle gears. When one wheel only moves, the spider spins the axle gear.
 
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