Do I have a high-speed axle ratio?

kingbee

kingbee

Senior Member
In my new-to-me '30 sedan it seems like the axle ratio is not standard.
At 60 MPH the tach indicates 2750 RPM, and the speedo is 6-7 MPH slower than my GPS.
Also seems to go faster in 1st & 2nd gears, with a typical shift to 3rd at 30 MPH.


What say ye?
 
oldredford said:
At 50mph with the standard 3.78 ratio your rpms should be around 3000. You may have a 3.25 ratio.
Click to expand...

:confused:


With a 3.78 rear and stock 21" or 19" tires, the engine should be turning 2100-2200 RPMs @ 50 MPH.

60 MPH is more like 2800 RPM.

This is described by the Torque - Speed - HP graph chart published in a 1930 sales catalog, and reprinted in a number of books on the Model A.

It also jives with what I have documented with my own Special Coupe (stock everything and 21" tires), using a test tach ( that connects to the spark-plug), and timing with a watch with sweep-second hand and using mile-markers ( every 1/10th mile).
 
Is the "60 MPH" from your speedometer, or from your GPS?

I'd jack up one rear wheel, shift to high gear, and count the turns on the hand crank to make the rear wheel make exactly 2 turns. That will be your ratio.
 
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