Large Tires
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More Turbo
Joined: Feb 2011
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From: Atlanta, Georgia
Large Tires
I am considering upsizing to something along the lines of 40" or higher tires...I have seen a couple things on here about "re-gearing". What does that mean exactly, and what happens if you DO NOT do it?
Hotshot
Joined: Aug 2006
Posts: 17,398
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From: Whittier, CA
Regearing refers to changing the gears in the differential to change the ratio. You probably have 3:73 gears now. You might change to 4:30 for example. The whole idea is to return the final drive reduction ratio to what it was before you increased the tire size. And remember, that if you have a 4WD, you have to do both front and rear differentials.
The manufacturer designed your total gear reduction, including transmission, differential, and tire diameter, so that your engine is turning at the most economical RPM at typical highway speeds. When you grossly change that total gear reduction, power and economy suffers. Especially since those larger tires and wheels are going to weigh a lot more than stock. If done right, regearing should get you back to the approximate OEM total gear reduction. You still have to deal with those heavy tires and wheels and the effects on steering, dampening, braking, and so forth.
Now I'll add my opinion, that a truck with 40" tires is a bit over the top, and you'll need so much lift that the truck becomes useless. But heck, it's your truck.
The manufacturer designed your total gear reduction, including transmission, differential, and tire diameter, so that your engine is turning at the most economical RPM at typical highway speeds. When you grossly change that total gear reduction, power and economy suffers. Especially since those larger tires and wheels are going to weigh a lot more than stock. If done right, regearing should get you back to the approximate OEM total gear reduction. You still have to deal with those heavy tires and wheels and the effects on steering, dampening, braking, and so forth.
Now I'll add my opinion, that a truck with 40" tires is a bit over the top, and you'll need so much lift that the truck becomes useless. But heck, it's your truck.
Posting Guru
Joined: Sep 2009
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From: Virginia beach, VA
You're going to need at LEAST 8" of lift for 40's, preferably 10" or more. Unless you're one of those cut-out flares kind of guys, then you can get away with like 4" of lift.
To answer your question, if you don't re-gear your truck will be slower and get worse fuel economy.
To answer your question, if you don't re-gear your truck will be slower and get worse fuel economy.
Junior User
Joined: Jan 2011
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your trans,mpg,and power are all going to hate you if you dont re gear. Just look at me i have a 5.4 stock gears and 37s i get about 7 mpg and my trans yells at me when im driving and the truck is as slow as a rowboat.
Posting Guru
Joined: Apr 2007
Posts: 2,402
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When you drive a truck with a manual transmission, you start out in first gear. When you add a larger tire you create stress on the drivetrain, its like starting out in third gear instead if first. Small changes in tire diameter are ok but when you make dramatic changes, such as going from 33 inch tire to 37, you need to change the axle ratio to reduce stress on the drivetrain. Hope that makes sense.
Elder User
Joined: Jun 2011
Posts: 668
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Agree. I would say 37s would be the largest I would go. Not only for the drive train. Also all of your stearing components up front .. bearing assemblys ect. Also when you get way up in lift the truck becomes less safe in my opinion. more apt to roll like a ball in an accident.
FTE Leadership Emeritus
Joined: Sep 2000
Posts: 23,174
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From: Stephensville WI
Lets say you have a truck with 32 inch tires and 3.73 gears. You want to put 40 inch tires on it. Using the Effective Ratio calculator, we find out putting 40 inch tires on your truck is the same as changing the differential gearing to 2.98:1.
That would suck for towing.
How do we fix it? 
Plugging the same numbers into the Equivalent Ratio calculator, we find out that you would need to change your differential gearing to 4.66:1 to keep the stock effective gearing while using 40 inch tires.
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Hotshot
Joined: Aug 2006
Posts: 17,398
Likes: 317
From: Whittier, CA
It also helps to know the commonly available ratios for these diffs.
Off the top of my head:
3:31
3:55
3:73
4:10
4:30
4:56
4:88 ??
5:13
5:38
The higher the numbers, the greater the gear reduction. You'll need to round off to one of these. Also keep in mind that with the PSD, it is better to err toward a lower engine RPM, than one that is too high. We make good torque down low.
Off the top of my head:
3:31
3:55
3:73
4:10
4:30
4:56
4:88 ??
5:13
5:38
The higher the numbers, the greater the gear reduction. You'll need to round off to one of these. Also keep in mind that with the PSD, it is better to err toward a lower engine RPM, than one that is too high. We make good torque down low.
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Posting Guru
Joined: Jan 2012
Posts: 1,176
Likes: 4
From: Indiana
I can tell you from experience-
I bought a truck that had 36 inch tires on it without a regear, and at 50-55 mph, it was a nightmare- kept jumping in and out of overdrive. It shifted at all the wrong times. Power-wise, it felt like a go-cart pulling a cement truck.
Another thing that was touched on by the others- your steering and brake components. New big fat tires with a re-gear only gives you new tires and new gears on the differentials. You have to make sure that your tie rod ends, ball joints, rotors and pads are in great shape- they will be seeing A LOT of new stress they didn't have before. And even with brand new parts- they will wear out faster with tires of the size you are talking about. The previous owner of my truck only went with 36's, but did so on factory original parts. I'm rebuilding the front suspension as we speak because of it.
No one is trying to tell you not to do it- It'll look pretty sweet. Just know what you are getting into. Big tires are not a "set it and forget it" proposition. You'll have to periodically inspect the systems to make sure everything stays cool for ya.
I bought a truck that had 36 inch tires on it without a regear, and at 50-55 mph, it was a nightmare- kept jumping in and out of overdrive. It shifted at all the wrong times. Power-wise, it felt like a go-cart pulling a cement truck.
Another thing that was touched on by the others- your steering and brake components. New big fat tires with a re-gear only gives you new tires and new gears on the differentials. You have to make sure that your tie rod ends, ball joints, rotors and pads are in great shape- they will be seeing A LOT of new stress they didn't have before. And even with brand new parts- they will wear out faster with tires of the size you are talking about. The previous owner of my truck only went with 36's, but did so on factory original parts. I'm rebuilding the front suspension as we speak because of it.
No one is trying to tell you not to do it- It'll look pretty sweet. Just know what you are getting into. Big tires are not a "set it and forget it" proposition. You'll have to periodically inspect the systems to make sure everything stays cool for ya.
Thread Starter
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More Turbo
Joined: Feb 2011
Posts: 531
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From: Atlanta, Georgia
Great information, realistically, I probably will not go with 40" tires, they are highly expensive and the suspension work is too extensive. May go with 37s...I only want to do around a 6 inch lift, so whatever I can fit with that! I have the stock 18" wheels, does anyone know how large those tires are?
Token Redneck
Joined: Jun 2007
Posts: 9,331
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From: Breckenridge, TX
37s can be an odd-ball for rim fitment. There's a lot of sidewall to work with that let's you put them on a lot of rim widths, but anything more than 12.50 (315mm) wide is really too much for a stock 8" rim.
Freshman User
Joined: Feb 2012
Posts: 49
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Not regearing will create unbearable fuel economy. And an insane increase in stress on the tranny and engine. Also. With that much lift you will need a new driveshaft. Another thing if ur going to tear apart your diffrentials, you might as well put lockers in
Freshman User
Joined: Feb 2012
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And another thing is you will have to change the angle of the axle so the joint where it meets the driveshaft will function properly. So youre looking at alot of work and money. I wouldnt go bigger than 37s if it was me. But its ur truck
Postmaster
Joined: Oct 2005
Posts: 3,620
Likes: 3
From: WARRINGTON, PA
I don't know.... If you are looking to put 40's on a truck, you should atleast have somewhat of an idea on what gears, differentials, lift kit sizes needed, and how things work. With that said, I hope you aren't doing anything more than driving it on the street. Sorry if that came out wrong.