Differfential Gear Ratio - Determining
 

Since I published this chart on a few web sites, several Model A'ers have asked, "What gear ratio does my car have." Another question often asked is, "How do I determine what the gear ratio is in my Model A?" So lets help these folks by posting your technique for determining the differential gear ratio in a running Model A.

Re: Differfential Gear Ratio - Determining
 

I jacked up the passenger side rear, removed the spark plugs, put the trans in 3rd, turned the engine hand crank and counted the turns of the rear wheel. I turned the engine by hand for several turns to reduce any error in guess-timation of the turns of the rear wheel. I then took the number of turns of the wheel and divided by two (becasue of the differential effect) and took that result and divided the number of turns of the engine crank by that result to get the approximate rear end ratio.
The possible ratios are discrete choices so you get to the correct ratio pretty easily.
10 turns of the crank gives about 5 and 1/4 turns of the rear wheel. 5-1/4 divided by 2 is 2-5/8. 10 divided by 2-5/8 is 3.8 which is close to the 3.78 rear end ratio.
I did this because my tachometer was giving me strange RPM's compared to the chart above. The tach was getting interference from an old condenser in the distributor.

Re: Differfential Gear Ratio - Determining
 

By the way, I have an overdrive that had to be in the non-overdrive position.
All things considered, I think a rear end ratio of 3.54 would be a good choice without having an overdrive. I have thought about changing the rear end ratio and eliminating the overdrive.
Also, thank you to Bob for posting so many useful and timely items on the forum.

Re: Differfential Gear Ratio - Determining
 

MikeV in Florida posted this in 2016. It worked easily for me.
Quote:
Lots of things can change in 80 plus years. Jacking one rear wheel up and turning the engine over by hand will let you determine the ratio. Easiest to pull your plugs and get your hand crank out,
Put it in 3rd Gear.
Make a mark on the jacked up tire at 12 o'clock and crank the engine over exactly 2 revolutions. When you hit exactly two revolutions, have a friend grab hold of the tire so it cannot move, or set the hand brake or whatever.

See where the mark on tire ends up:

Between 11 and 12 o'clock, ratio is 4.11
Between 12 and 1 o'clock, ratio is 3.78
Between 1 and 2 o'clock, ratio is 3.54
Here is the info I have;
Jack up one rear wheel.
Pull your plugs and get your hand crank out,
Put it in 3rd Gear.
Make a mark on the jacked up tire at 12 o'clock and crank the engine over exactly 2 revolutions. When you hit exactly two revolutions, have a friend grab hold of the tire so it cannot move, or set the hand brake or whatever.

See where the mark on tire ends up:

Between 11 and 12 o'clock, ratio is 4.11
Between 12 and 1 o'clock, ratio is 3.78
Between 1 and 2 o'clock, ratio is 3.54

These are from notes of the procedure that I have taken from another Barner's post.

Re: Differfential Gear Ratio - Determining
 

Why is the engine cranked 2 revolutions?

Re: Differfential Gear Ratio - Determining
 

Move car so valve stem is up( I use rf wheel), put in 3rd, make mark on crank pulley, roll car ahead watching mark on pulley till valve stem is up again, a little over 4 turns =4:11, 3,3/4==3:78. If you want to be lazy and leave the plugs in rocking the wheel bounces against compression and it will move easier than just pushing

Re: Differfential Gear Ratio - Determining
 

How does that even work? If you had a 4:1 ratio and you turned the crank two revolutions, you would only have turned the tire half a turn.

Re: Differfential Gear Ratio - Determining
 

Because one back wheel is held stationary, the other turns twice as much as it would if both wheels were turning. In the example you gave, the back wheel would turn nearly one full turn with 2 turns of the crank.
This is the method I use and have done so for years.

Re: Differfential Gear Ratio - Determining
 

OK, got it. He didn't mention only one tire was off the ground. Think I'll just stick with Kurt in NJ way, which is the same way I have always done it.

Re: Differfential Gear Ratio - Determining
 

I added jacking up one rear wheel which was inadvertently left out the post #4.