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DogRiver101 12-28-2006 10:16 AM

Moving from 31's to 33's
I'm currently running 31x10.5 on a bone stock suspension and just acquired a Rusty's 2" suspension lift. I'm considering moving up to a set of 33 x 10.5 (want to keep my stock wheels), but I'm concerned about power loss. I have a 4.0, with an auto trans, and 3:73 gears, which is quite peppy with the 31's. Does anyone know how much power loss I can expect if I make this change?

Siggie 12-28-2006 11:16 AM

I'm a newbie to Jeeps, but I wrenched a lot of cars. First, let me reframe the question: You're increasing the rolling diameter of your drive wheel and you want to know the affect that it will have on torque and horsepower.

The first thing that you'll need to do is to calculate what the tire change will do to your engine RPM at a given speed. Attached is a link that allows you to plug in values for wheel diameter, final drive ratio, transmission gear ratio to find out what the RPM difference is.

Your next step is to google to get the engine horsepower and torque map for the 4.0 engine. And voila! At any given speed you can calculate what the change in torque and horsepower will be -- and you'll have the absolute values.

In your case, I'd guess there would be about a 2-3% increase in torque (the quantity that makes you feel like your being pushed back into your seat) and about a 5% DECREASE in horsepower (the quantity that sells SUVs) :D That's kind of a shitty and innaccurate estimate based on the change in tire diameter.

A nice byproduct of the change will be that your cruise RPM will go down (which means your speedometer will need recalibration) and you'll have less engine noise (which may be more than compensated for by more tire noise).

kg4kpg 12-28-2006 12:06 PM

If you want your speedo to read correctly just order the little plastic gear from 4wd Hardware, Jeep or other companies. 4wd catalogs have a scale to determine which one you would need.

You will loose some power which can be helped out a number of ways. New ring and pinion gears for one. When I went from 31's to 33's on my 4 banger I kissed 70mph goodbye, especially on windy days. Used more gas as well. Eventually I am switching from 4.10's to 4.56's when I put my current jeep on 33's. I'm sure others here will give plenty more advice too. Enjoy.

DogRiver101 12-28-2006 12:35 PM

thanks for the link. How will the larger tire increase my torque?

Siggie 12-28-2006 12:55 PM


Originally Posted by DogRiver101 (Post 41222)
thanks for the link. How will the larger tire increase my torque?

From my sadistic professors:

Torque = Force X Moment Arm

The basic physics are the same in any application. Think of a socket wrench. You've got the wrench on a rusted big bolt and you pull as hard as you can but the bolt won't budge. Go find a 3 foot long steel tube and put it over the handle of the wrench -- now pull. The bolt breaks free. (or if it's my garage, you broke the head off the bolt!)

In that example, you increase the moment arm, or leverage. More torque is produced. The output driveshaft of your engine (driving the planetary gear in your final drive) is JUST like you pulling on that wrench, only instead of untorquing a rusty bolt, it is torquing a wheel that is trying to overcome the inertia associated with a 4000lb Jeep being made to accelerate (or overcome the resitance of maintaing speed). Increase the moment arm, you have more torque.

Now this is a little tricky. If you checked the engine map, you noticed that the ENGINE produces more torque as RPM increases. Therefore, at a given SPEED, your change to the 33 inch tires reduces the engine RPM. Makes sense -- right? A bigger tire goes a greater distance per revolution than a smaller one. So at a given speed, your torque with the 33 inch tires is lower than the torque produced AT THE SAME SPEED WITH 31's. Of course, as John Force and any race car driver knows, the thing to do is to run the engine at a higher RPM! :D Overall, increasing the tire diameter will increase your capacity to make torque, at the expense of running the engine faster to access that additional capacity -- the stuff that makes your head snap back when you dump the clutch, or the ability to muscle up a steep incline.

mrbigjeep 12-28-2006 05:45 PM

i run 32's and 3.07's. you should be fine with 33's and 3.73's. especially since you'll be runnin a 33" tire that is only 10.5" wide.

a guy on another forum just went from 32's to 33's and he couldn't tell a difference. he went with the 33x10.50's and had the 32x11.50's before. he said the loss of going from 31x10.50's to 32x11.50's felt bigger than when he went from 32x11.50 to 33x10.50.

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