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EFV-8 Club Forum / General Ford Discussion / swapping gears

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Discussion Topic: swapping gears

41fatty

07-14-2025 @ 1:57 PM

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if I change my ring and pinion from 3.78 to 3.54 will i need to change my spider gears ( 11 tooth) and/or my axles( 16tooth)? will they mesh? they appear to be in fine condition, other than the usual seals bearings races etc.

juergen

07-16-2025 @ 5:26 AM

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The ring and pinion are compatible for the same eras like 1937-48. Of course you will need a lower teeth turtle.

41fatty

07-16-2025 @ 11:39 AM

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what's the difference between turtles and spiders? I am not sure that I need to change the 4differental pinions that attach to the spider.

07-16-2025 @ 11:57 AM

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"turtle" - Reference to the speedometer gear which you will have to change if the ratio of the rear end is changed...

A pinion with 11 teeth and a ring with 39 teeth will give you a rear with a 3.54 to 1 ratio...

This message was edited by MG on 7-19-25 @ 9:53 AM

41fatty

07-16-2025 @ 4:34 PM

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thank you, duly noted, but; what if ( and when) i run on 16 in. wheels, will there be much of a difference? keep in mind I was always happy running on 3.78 configuration and am still looking for the same 9-34 ring and pinion but at what cost?

41fatty

07-16-2025 @ 6:30 PM

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correction, I meant 15in. wheels, opposed to 16

JayChicago

07-17-2025 @ 10:16 AM

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If I'm not missing something, here is an approximation:

16 " wheel plus 6 " tire = 28 " diameter X 3.14 = +/- 88 " circumference, travels 88" with each tire revolution.
15" wheel plus 6" tire = 27" diameter X 3.14 = =/- 85" circumference. Travel per revolution is 3.5% less.
Your existing 3.78 gear ratio, plus 3.5% more engine/drive shaft turns per travel distance = 3.91

Going to 15 " wheels is equivalent to changing gear ratio to 3.91

JayChicago

07-18-2025 @ 7:51 AM

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Whatever differential gear ratio you have, a change to 15" wheels will give the same 3.5% more engine turns per distance of travel.
With a 3.54 gear, going to 15" wheels is equivalent to a 3.66 gear.

juergen

07-19-2025 @ 6:13 AM

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And switching to a 3.54 from a 3.78 will give you a 6.3% lower engine speed (or reading in mph). Together with the 3.5% in the opposite direction will result in a 2.8% change in speedometer reading. At 60 mph this is in the ballpark of speedo error.

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