Who Here Has Done New Gears w/ 37's? - Ford Powerstroke Diesel Forum
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Old 07-22-2011, 08:06 AM
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Who Here Has Done New Gears w/ 37's?

Believe the time has come to do gears on my truck. It has 37x13.50-22 on it right now, but only 3.55 factory gears. Didn't bother me too much until I started towing a new boat that's around 5K pounds on a regular basis. Under 50mph, not bad, but when trying to maintain highway speeds up any small incline, EGTs rise a little too rapidly and boost to over 20psi just because the engine is lugging.

Thinking of going to 4.30's, but possibly 4.56? Whose done a gear swap and whats you impressions?
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Old 07-22-2011, 02:37 PM
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you need gears if you tow. you can google "gear calculator" and many will come up with a grid of equivalent tire size versus gears. 3.73 gear on 31.5s(stock) nearly exactly equals 4.56 with 37s. If your trying to get as close to stock as possible, 4.30s be best to match your 3.55 gear ratio. best mod you can do, brings back all your power. Throw a Detroit locker in the rear while you’re at it, you will be thanking me
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Old 07-25-2011, 06:06 AM
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My truck came factory w/ 3.31. I went to 4.10 w/ a locker and couldn't be happier. Around 1900 rpm at 70mph
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Old 07-25-2011, 05:38 PM
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anyone wanting 4.10's i would trade mine for 3.55's. stock tires w/ 4.10's 2100@ 70 mph... these gears are killin me. And more importantly i got smoked by a vette on the highway cause truck wont do more than 95 ish.
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Old 07-26-2011, 07:14 AM
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Thanks for the input y'all. Leaning towards the 4.30's right now. But also consider the 4.56's, as my fiance's grandparents just gave us a 31.5' Airstream that they no longer use.

Hey JetJock, what kinda RPM's are you seeing at 70mph? And what kind of mpg's are you seeing with the 4.56's?
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Old 07-26-2011, 08:18 PM
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I have 3:55 rear end and my rpms are right at 2000 @70.
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Old 07-30-2011, 03:58 PM
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If you want to keep some of your fuel mileage when you aren't towing I would go 4.10s or 4.30s. If you go off-road at all or even just don't like having to use 4 wheel drive cause you get stuck on wet grass, I second the rear locker install at the same time. I had a Detroit installed in my '99 when I went to 4.10 gears and absolutely loved it.
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