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?

 

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.

 

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.

 

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.

 

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.

 

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.

 

What you can do is use the automotive calculators on this site to figure out what you need to do. Automotive Calculators offer to you by Ford Trucks Enthusiasts | www.ford-trucks.com

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.

 

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.

 

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.

 

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?

 

And with the numarically higher gear it (normaly) has less material and can be weaker.

 

Stock 275/70R18s are about 33x11, depending on the brand of the tire. Stock tires ranged up to around 34", and I'm running 34.5" (actual) with no lift. 37s will probably fit on an 05+ truck with 4.5" or more of lift , depending on how tight inboard the backspacing of the wheels keeps the tires. Very generally speaking, the further out the midline of the tire, the more lift you need to clear it. Icon makes a complete 4.5" kit that can be boosted to 5.5" with a lower spring spacer. That's the way I'd go,, get the 4.5" and see how it looks, then maybe slide an extra inch in if it needs it. The next step up is a 7"/8" kit, which costs roughly the same as the 4.5" kit, but you'll be looking more seriously at supporting mods in the driveline, steering, and and brake lines once you start talking that tall.

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.

 

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

 

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

 

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.