Choosing a Gear Set

Recently I read an article in the July 15, 1919 Ford Service Bulletin that explained torque vs horsepower in simple terms. Here is an excerpt from the article.

An engine is like a man in that it has two strength settings. A man may be able to lift a weight of 100 pounds, but he cannot run with it. With a load of ten pounds he can run. The amount an engine can “lift” is termed “torque”, while the rate at which it can carry (turn over) that weight is termed “horsepower’ The torque and horsepower increase with R.P.M. (revolutions per minute) for a time and then gradually decrease.

What has this to do with rear end gear ratios? The gear ratio is similar to “weight” in the above example. A 3:1 gear ratio puts a greater load on the engine and requires more torque and horsepower to get up to speed. A 4:1 ratio burdens the engine less but requires more RPM to get attain speed.

Said another way. With a 3:1 ratio you may not have enough power to go fast and with a 4:1 ratio you may not have enough RPMs to go fast. Ford’s standard 3.63: 1 ratio is a great compromise.

The three most common ratios are: 3.63:1, stock ratio (11 tooth pinion - 40 tooth ring gear), 4:1 (10 tooth pinion - 40 tooth ring gear), 3:1 (13 tooth pinion - 39 tooth ring gear).

Here is a list of some of the factory and aftermarket gear ratios available for a Model T.

3:1, 3.07:1, 3.25:1, 3.33:1, 3.63:1, 4:1, and 4.44:1. In addition to these ratios the TT truck had a different rear end with different ratios.

Using the data associated with the service bulletin article I came up with this chart.