Earlier in this thread I wrote:
Originally Posted by Sherpa
I wouldn't suggest going any taller than 31s with this engine/gear setup.
I offered this advice based on my own real-world experience of having run two different 2.5L 5-speed TJs with 4.10 axle gears and 31" MT/R tires. There are a number of factors going into this advice; I'm going to expound on one of them below.
Originally Posted by hmiller
so duratrac 33's would be smaller than most 33's?
It is very true that tire sizes do vary by manufacturer (and even tire model). Because of this, some feel it is important to research the exact diameter of a tire before purchasing it. While the diameter is certainly a good specification to know about a tire, it isn't the only important one--not by a long shot. As your tires get larger, their weight
becomes more and more of a critical issue.
Adding weight to a vehicle is always detrimental to its performance; acceleration, fuel economy, and braking distance will all be negatively affected. However, all weight is not equal in this regard. The worst weight you can add--the weight that has the biggest impact on the three factors I just mentioned--is unsprung rotating weight... in other words, your tires and wheels.
For example, drive your Jeep with only you inside and note how quickly it accelerates to a given speed. Now throw 200 lbs of weight into the back and try it again. The time required will increase. Now remove that dead weight from the Jeep, and then install a new set of wheels and tires where each mounted wheel/tire weighs 50 pounds more than the one you removed. Perform a third acceleration test. The time required will be even longer than before despite the Jeep having the exact same curb weight in tests 2 and 3.
This is why you have to more than compensate for larger tires when you regear. I'll use my own Jeep as an example. I used to run 31" tires on my 3.73 gears, and I was happy with its acceleration and performance. I knew I'd be upgrading to 35" tires, and I'd want to regear to compensate. On paper, going from 31s to 35s is a 13% increase in diameter. To stay equal, you'd want to increase the gearing by the same amount... right? Multiplying 3.73 by 113% equals 4.21, so if I went with 4.56 gears then I would certainly
be in good shape, right? That would be a gear increase of over 22%, or nearly double what the math says I require.
And yet my acceleration (and braking) are both noticeably slower than before I made my modifications. How can that be? A large part of the answer is that each 35" wheel/tire combo weighs 80 pounds. I even took weight into consideration when I bought the new wheels and tires, selecting some of the lightest 15x8 wheels I could find as well as choosing the BFG KM2 specifically because it was 7 pounds lighter (per tire) than the equivalent MT/R.
You may wonder how heavy my 31" tires were. They were mounted on Jeep factory 15x7 alloy wheels, and each mounted combo checked in at 61 pounds. But that's only 20 pounds each, you say. Yes, but all that extra weight has been added at the furthest point from the axis of rotation. This means my engine, as mighty as the 4.0L is, must struggle to get these four masses rotating every time I start moving. (And then my poor brakes must also struggle to bring this rotating mass to a halt every time I press the brake pedal.)
In short, rotating mass is a colossal pain in the patoot. Its negative effects worsen as you add weight further and further away from the axis of rotation. You would do well to keep this weight as low as possible.
Originally Posted by bobbylx
This is half a 2 inch budget boost with 33x10.50x15 BFG AT's.
It's a 4cyl 5spd with stock 4.10s. I run 65-70mph on the highway no problem
Maintaining a set speed on flat ground is not difficult; getting up to said speed (or even maintaining it on a steep hill) is where you'll see the impact of the larger tires. I know for a fact your acceleration has been affected, because I saw it when I only ran 31s.
Originally Posted by Enemy
sorry to highjack but what would be the biggest tire to run on a 4.0? I currently have 31's and want 33's.
You say your TJ is an X model, suggesting it has a Dana35 rear axle. I would suggest you not go any larger than a 33" tire on that axle, period--it simply isn't strong enough to handle it.