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04-03-2021, 07:27 AM | #1 |
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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.
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Bob Bidonde |
04-03-2021, 11:02 AM | #2 |
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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. |
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04-03-2021, 11:07 AM | #3 |
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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. |
04-04-2021, 09:39 AM | #4 |
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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. Last edited by GPierce; 04-04-2021 at 07:46 PM. Reason: Added jack up one rear wheel. |
04-04-2021, 10:15 AM | #5 |
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Re: Differfential Gear Ratio - Determining
Why is the engine cranked 2 revolutions?
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Bob Bidonde |
04-04-2021, 10:28 AM | #6 |
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Re: Differfential Gear Ratio - Determining
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04-04-2021, 03:24 PM | #7 | |
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Re: Differfential Gear Ratio - Determining
Quote:
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04-04-2021, 03:59 PM | #8 | |
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Re: Differfential Gear Ratio - Determining
Quote:
This is the method I use and have done so for years.
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04-04-2021, 04:38 PM | #9 | |
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Re: Differfential Gear Ratio - Determining
Quote:
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The Master Cylinder Enjoying life at the beach in SoCal... Last edited by The Master Cylinder; 04-04-2021 at 08:51 PM. |
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04-04-2021, 07:48 PM | #10 |
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Re: Differfential Gear Ratio - Determining
I added jacking up one rear wheel which was inadvertently left out the post #4.
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