gears.
Senior User
Joined: Jan 2004
Posts: 174
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From: Illinois
I had 4.56 and 38 inch ssrs on my 6.0. The gearing was a little lower than it ran stock, higher highway rpms. I think with 40`s it would be just about perfect. You will def. notice a big increse in power and it makes it alot easier on that torqueshift.. 
Posting Guru
Joined: May 2004
Posts: 1,414
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From: stoneboro, pa
i have 4.56's in my bronco with 40" swampers. its the perfect match. speedo is pretty close. the bronco and ur SD had about the same tire size and gears factory, so id imagine thats ur best bet
Cargo Master
Joined: Jun 2003
Posts: 2,203
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From: Carlsbad, California
Depends, do you want to get back to the equivalent stock ratio with the 3.73 and the 265/75R16s or do you want something lower?
The stock tires have a calculated diameter of 31.65 inches so if you divide 40 in by 31.65 and multiply the result by 3.73 you come up with 4.714.
So you could go with the 4.56 or a 4.88 if you want something a tad lower geared.
The stock tires have a calculated diameter of 31.65 inches so if you divide 40 in by 31.65 and multiply the result by 3.73 you come up with 4.714.
So you could go with the 4.56 or a 4.88 if you want something a tad lower geared.
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