gear ratio help!!!! - Diesel Truck Forum - TheDieselGarage.com
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post #1 of 8 (permalink) Old 09-30-2010, 03:38 PM Thread Starter
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Exclamation gear ratio help!!!!

i have a 1991 f350 7.3 international. it has a 4 inch suspension lift with 33 in tires. i just purchased new 36 inch tires and i am looking to put a 3 inch body lift on the truck. do i need a different gear ratio for the 36s? or will it not effect the truck at all? please help.
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post #2 of 8 (permalink) Old 09-30-2010, 11:07 PM
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Short answer, YES, or recommended.

It will affect the truck adversely, you will notice a decrease in power, not only due to the tire height but weight and rolling resistance increases too. Although you will be able to achieve higher top speeds (if you have the power) You speedometer will be way off reading slower than you are actually going too. You may need to start using low range more when off road too.

All this is not to say it cant be done or render the truck useless but it will be noticable. If you do towing you will want to re-gear. I think these trucks came from the factory with around a 31" tire. You may not have noticed as much of a difference going to a 33" but it will be a bigger difference from 33" to 36". Afterall N/A IDI's were never power-house engines, even with a turbo.

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post #3 of 8 (permalink) Old 10-01-2010, 02:35 AM Thread Starter
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Thanks for the help. What ratio do you suggest?
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post #4 of 8 (permalink) Old 10-01-2010, 06:55 AM
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I say no. I run 37" tires on stock gears and my mileage went up. You have enough torque with the diesel to do it. When loaded and pulling you may notice it a little but for the price I wouldn't bother.
FYI mine has 4:10 gears factory. I need more info on yours.


You should clear 36" with the 4" unless it's 2x. I ran 37" with a 4" and all I had to do was trim the front bumper a little.

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post #5 of 8 (permalink) Old 10-01-2010, 01:28 PM Thread Starter
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I only have 3:55 gears. Half the people tell me I need to change it and the other half tells me to leave it. I just want whatever is best for the truck.I really don't want to lose gas mileage and I don't want to have to floor it just to get it to move out of a stop sign. What do you think?
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post #6 of 8 (permalink) Old 10-03-2010, 04:16 AM
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Id try them and see how it does. If it seems to dog more then spend the money on gears.

What transmission does it have?

95 bronco 94 idi turbo, intercooled, maxed out pump, custom compressor, custom downpipe, 12* timing, oba, d60, 10.25, e-locker, exocage, zf5, etc.....
2011 f-550 6.7L 12ft service box, shop on wheels
80 merc 300sd, hx35, intercooled, straightpiped, modified pump, egr delete, etc.....
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post #7 of 8 (permalink) Old 10-04-2010, 03:38 AM
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^ good advise, I would say go ahead and try it too, but dont feel bad when you have to drop the money for new gears. If you feel you need to re-gear then 4.10's would be a close match to the factory tire size and 3.55's. If you leave them alone you have basically turned your 3.55's into 3.08's with the 36" tires assuming factory size of 31"

You should fill out your sig line with truck specs

My motto: Be nice, be polite, but have have a plan to kill everyone that you meet.

94 F250 IDI turbo, auto 2wd.. ATS housing, 3" down, 4" exh, guages. 104k miles got totalled by a damn mustang (parts now).

98 12v, 4x4 quad cab. auto with BD T/C & V/B, Banks Big head, Exh brake, AFC modded, 16.5ish*, KDP gone, 60lb springs, 4K-GSK, BHAF, pyro, trans, and boost guages. 4" straight pipe,

72 AM General M35A2, LDT465-1C "Whistler" Turbo, PTO winch.
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post #8 of 8 (permalink) Old 10-09-2010, 02:02 AM Thread Starter
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Thanks for the help. Its an e4od
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