WHAT IS DIFFERENCE IN RPM AT 75 MPH BETWEEN 3:31 AND 3:55 - Ford Powerstroke Diesel Forum
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post #1 of 3 Old 05-22-2011, 10:50 AM Thread Starter
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WHAT IS DIFFERENCE IN RPM AT 75 MPH BETWEEN 3:31 AND 3:55

Hello. I am going to order a 350 cc long box shortly and am just trying to figure out which rear end I want. Tow pretty much daily but nothing above 6000 lb on a normal basis. Curious on how much the difference is on rpm and or mpg between these two different gears when empty as either one will work fine for what I tow. Usually run about 75 on highway when I'm not towing. Just for comparison lets figure stock 18" wheels. Thanks for any input.
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post #2 of 3 Old 05-23-2011, 07:31 AM
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Based upon a gear chart that I saw for the 6.7's, if you order a 3.31 with 18" wheels and then put on 20" wheels the difference is about 100 rpm across the board.
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post #3 of 3 Old 05-24-2011, 08:06 PM
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If the tires are the same size on both trucks the rpm of the 3.31 will be 93% the rpm of the 3.55. With 275/65r20" wheels that works out to 1760 rpm on the 3.55 and 1637 on the 3.31 at 75 mph. With 275/70 r 18 " tires it works out to 1808 rpm with the 3.55 and 1682 with the 3.31.

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