The Ford Barn

marty in Ohio · 12-10-2014, 02:26 PM

I'm trying to determine the gear ratio in my '30 town sedan. I removed the plugs, jacked up one wheel and marked it at the 6 o'clock position, and put the transmission in 3rd gear. I turned the engine over with the crank two times (like it was described to me a year ago on Ford Barn) and the tire mark ended up in the 5 o'clock position. When I tried again and only turned the crank one revolution the tire mark ended up between the 11 and 12 o'clock position. What am I doing wrong? One wheel up - two turns on the crank - third gear.

Tom Wesenberg · 12-10-2014, 03:00 PM

Sounds like you have the common 3.78 ratio. Remember with one wheel up and the other wheel locked, the wheel that turns goes twice as fast due to the differential action.

In other words if you turn the hand crank 3.78 turns, the wheel off the ground should make exactly two revolutions.

Patrick L. · 12-10-2014, 03:45 PM

Do you have room to roll the car and count the turns of the pulley ?

marty in Ohio · 12-10-2014, 05:42 PM

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Patrick,
Not really. The car is laid up for the Ohio winter (we call it hibernation) and I can't really roll it. Another Model "A"er also wants to find out what gear ratio he has because he has a different ring and pinion to install.

12-10-2014, 02:26 PM	#1
marty in Ohio Senior Member Join Date: May 2011 Location: Elyria, Ohio Posts: 806	One turn or two turns? I'm trying to determine the gear ratio in my '30 town sedan. I removed the plugs, jacked up one wheel and marked it at the 6 o'clock position, and put the transmission in 3rd gear. I turned the engine over with the crank two times (like it was described to me a year ago on Ford Barn) and the tire mark ended up in the 5 o'clock position. When I tried again and only turned the crank one revolution the tire mark ended up between the 11 and 12 o'clock position. What am I doing wrong? One wheel up - two turns on the crank - third gear.

12-10-2014, 03:00 PM	#2
Tom Wesenberg Senior Member Join Date: May 2010 Location: Mpls, MN Posts: 27,582	Re: One turn or two turns? Sounds like you have the common 3.78 ratio. Remember with one wheel up and the other wheel locked, the wheel that turns goes twice as fast due to the differential action. In other words if you turn the hand crank 3.78 turns, the wheel off the ground should make exactly two revolutions.

12-10-2014, 03:45 PM	#3
Patrick L. Senior Member Join Date: May 2010 Location: Largo Florida Posts: 7,225	Re: One turn or two turns? Do you have room to roll the car and count the turns of the pulley ?

12-10-2014, 05:42 PM	#4
marty in Ohio Senior Member Join Date: May 2011 Location: Elyria, Ohio Posts: 806	Re: One turn or two turns? Sponsored Links (Register now to hide all advertisements) Patrick, Not really. The car is laid up for the Ohio winter (we call it hibernation) and I can't really roll it. Another Model "A"er also wants to find out what gear ratio he has because he has a different ring and pinion to install.