I think I have this gear info all figured out!! - Ford Powerstroke Diesel Forum
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Old 08-21-2013, 10:54 PM
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I think I have this gear info all figured out!!

The only thing that worried me about buying my truck was the 3.3 gears, knowing that I wanted to put 37's on it. I did a lot of reading and lots of people said it was bad, but those that had it usually said no problem.

If my reading is correct the following calculations I made are very informative information that should clear up a lot of discussions that have "answers" all over the place.

This shows 1st through 6th gears (with their ratios) with stock tire height (275/70/18... 33" tall) in the 3.31 and 3.55 ratios. Then it shows the effective gear ratio if 37" tires are added, compared to a stock tire.

3.31 3.55 3.31 3.55 3.73 3.9 4.1 4.3
33.2 33.2 37 37 37 37 37 37
1 3.97 13.14 14.09 11.79 12.65 13.29 13.89 14.61 15.32
2 2.32 7.68 8.24 6.89 7.39 7.76 8.12 8.54 8.95
3 1.52 5.03 5.40 4.51 4.84 5.09 5.32 5.59 5.86
4 1.15 3.81 4.08 3.42 3.66 3.85 4.02 4.23 4.44
5 0.86 2.85 3.05 2.55 2.74 2.88 3.01 3.16 3.32
6 0.67 2.22 2.38 1.99 2.13 2.24 2.34 2.46 2.59

Lot's of people think they need 4.88 or some super low gearing, but they are trying to calculate on rear end gearing alone and not final drive gearing. For me to get back a stock 3.31 ratio with my 37's it would only require a 3.73. To equal an effective 3.55 stock ratio a 3.9 is super close (or 4.1).

I have two points of view in trying to decide whether to re-gear or not. 1- although 1st and 2nd are diminished the rest of the gearing is easily overcome by simply having the tranny downshift 1 gear (so re-gearing seems to just effect starting off from a stop)??? 2- If I did swap gears I was thinking 4.3... this would give me better than stock gearing which would increase power without being too low and it could help compensate for the added weight of the "rotating mass" of the larger tires.

Let me know if this math is correct or if I'm going down the wrong road. I have been drinking tonight so this could be genius or lame.

Edit... the chart didn't quite paste right... 1st column in gears, 2nd their ratios, 3rd 3.3 final ratio, 4th 3.55 final ration, and each corresponding column is effective ratio with 37's... just shift the top two rows to the right some

Last edited by Z28Johnny; 08-21-2013 at 10:56 PM. Reason: paste did not go right
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Old 08-21-2013, 11:04 PM
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Old 08-21-2013, 11:05 PM
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ha ha!! I got it...
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Old 08-23-2013, 07:32 AM
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You have it right going from the stock 33's and 3.31 gears to 37's and 3.73's will be pretty close to the same effective gear, the only other thing you have to consider is that the weight of the tire/wheel combo of 37's is a lot more so it will still be a little slower if you just go with a 3.73. That being said I plan to regear from my 3.31 to 3.73's instead of 4.10's so I can help keep decent highway mileage. The 3.73 regear will actually increase your around town mileage if you're running 3.31's on 37's now because it will be so much easier to turn them.
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