New Gears for 38 Inch Tires? - Ford Powerstroke Diesel Forum
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Old 05-27-2014, 08:11 AM
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New Gears for 38 Inch Tires?

I'm using stock 3.55 gears for 38's right now. Do I need to change gears? I don't necessarily feel a need to, but I don't want to harm anything if that's a possibility. Any thoughts?
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Old 05-27-2014, 09:46 AM
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Wont harm anything except mileage, alot of guys will tell you the trans gearing makes up for the higher gears. I am by no means a gear master but from previous vehicles Ive had rule of thumb was usually 4.10s for 37" tires, but theres success stories with these trucks and guys running 38's with 3.55s and 3.73s.

What did you do about getting the speedo correct? I havent looked into it lately but last I had heard it wasnt possible over 34"?
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Old 05-27-2014, 10:14 AM
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There is a member on here with some Hypertech inline module that gets plugged in and can correct the speedo for any tire size but as far as I know with the H&S you can only correct up to 34" tires as well. I'd say re-gear Dantron but that's just me. If you don't plan on towing or going to the mountains then you should be fine with the 3.55s. If I go above 35" tires I would re-gear but that's because I live in a hilly arear and want to tow with my truck.
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Old 05-27-2014, 10:33 AM
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Yep. The computer still won't adjust for anything bigger than 34's. You have to buy a Hypertech or Pro Comp in-line calibrator, for like $200. Not worth it to me. I don't really monitor my fuel economy and a cop will usually give you a warning if you play dumb saying that you had the cruise control set to the speed limit. I will most likely re-gear to 4.10's one day, but it's not a high priority on my to do list (turbo first!).
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Old 05-27-2014, 11:40 AM
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You can easily run the stock 3.55s. speedo will be roughly 2-3mph off around 45+. The ProComp tool does nothing over 34", there is apparently a code issue from Ford that limits this axle ratio/tire combination (spoke to Edge and SCT, same answer).
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Old 05-27-2014, 11:52 AM
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Quote:
Originally Posted by lowmpg View Post
You can easily run the stock 3.55s. speedo will be roughly 2-3mph off around 45+. The ProComp tool does nothing over 34", there is apparently a code issue from Ford that limits this axle ratio/tire combination (spoke to Edge and SCT, same answer).
You're talking about the OBD2 plug in module, I was talking about one that gets plugged into your gauge cluster. There is a member on this forum that has his speedo corrected for 38" tall tires.
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Old 05-27-2014, 11:58 AM
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Wow, yeah I was talking about the 200.00 unit here: http://www.summitracing.com/parts/ex...05-1/overview/
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Old 05-27-2014, 12:06 PM
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Yeah. The in-line speed-o-meter calibrator is hardcore and will do the trick, but (+/-) 5 MPH isn't that big of a deal to me. As far as the 3.55's, I haven't been able to tell if it's lacking because I don't have anything to compare it to. I just don't want to harm the transmission or something.
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Old 05-27-2014, 12:09 PM
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Hypertech has one as well:
Hypertech 730120 In-Line Speedometer Calibrator Module
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Old 05-27-2014, 12:14 PM
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I think that's something I'll get when I switch gear ratios in a few years..
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