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Old 03-16-2011, 08:55 AM   #8
Special Coupe Frank
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Join Date: May 2010
Location: Northeast Penna
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Default Re: Borg Warner R10 overdrive conversion for Model A?

FYI on rear-end ratios: most 1930's - 1940's cars that had OD from the Factory had ring & pinion ratios in the 3.9 - 4.11 - 4.3 - 4.6 territory ( 16-17" tire sizes average ).

That seemed to be a good compromise between OD for the Highway without making them too "doggy" on hills...

For example, take a garden-variety 1940 MoPar six with a 4.11 rear, and pop a 27% B-W OD behind it, and get a resultant final-drive ratio of 3.0003 : 1 - pretty tall gears indeed, especially for a 90 to 110 HP six.

Put that same 27% B-W OD into a Model A with stock 3.78 rear, and you get a 2.759 : 1 effective final-drive ratio... very tall indeed.
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