Modifying the TJ, Part 1: Tire size choice

[Lawrence]

Here is an abreviated description of the TJ as it stands today:
2004 Sport, 4 speed auto, 4.88 gears, 35x12.5x17 Toyo Open Country MT, on 17x9 rims.

I was just able to borrow a set of 33x12.5x15 BFG MT from a neighbor and I did some testing as far as rpms.

First impressions were that I forgot how fun it was to drive the Jeep, it was very quick, even with 4 people going up the mountain by where I live. It pulled very nicely and the steering feel was not as “heavy” (for lack of a better word). Then, we dropped off 2 people, and my neighbor and I went for a drive. Again, the TJ was very quick getting up to speed from a dead stop. We then went on the freeway as I wanted to check the rpms out, and also to make sure I wouldn't be caught between gear at a certain speed. Running the 33s with my 39-tooth speedo gear, I knew the speed would be slightly off, so we brought a GPS. Here are some numbers:

OD ON
60 mph Speedo - 57 mph GPS - 2,000 rpms
65 mph Speedo - 62 mph GPS - 2,250 rpms
70 mph Speedo - 67 mph GPS - 2,350 rpms
75 mph Speedo - 71 mph GPS - 2,500 rpms
80 mph Speedo - 76 mph GPS - 2,650 rpms

OD OFF
60 mph Speedo - 58 mph GPS - 2,900 rpms
65 mph Speedo - 62 mph GPS - 3,150 rpms
70 mph Speedo - 68 mph GPS - 3,350 rpms
75 mph Speedo - 71 mph GPS - 3,650 rpms

The Speedo mph and rpms are obviously approximate since I was reading off the gauge cluster as I didn't have a Scan Gauge II.

With the OD on, it seems to me that the Jeep was much "happier" at those rpms, rather than the rpms with my 35s. Going up the Glendale freeway (I-2) at 70 mph, I could watch the speedo needle drop like a fly with my 35s. With the 33s it held much better, only toward the end I kicked the OD off to give it a little boost. All in all, with the OD on, I think it ran better and I give it two huge thumbs up.

The only reservation I had was running with the OD off. The rpms were a bit more than I wanted. On the other hand, I won't be looking to break any speed record with the trailer behind (yes I know the speed limit thingy, blah, blah, blah), and at 60 mph the rpms seemed to still be fairly reasonable.

The part that sucks is that I have a brand spanking new set of 35s, but it is what it is. Overall, it looks like 33s will be a better choice and that I will be happier with the performance while going up a long grade or to the mountains.

Lastly, I have a line on a set of five 305/65/17 (33x12.5) Pro-Comp Xtreme MT for $600. I am considering getting them and I would like to know if anyone has any experience with those. I am not a big fan of Pro-Comp, but some people tend to say the Xtreme are the only good ones. To quote someone, the question is “are they good, or do they suck less than others?” Another choice would be the new BFG KM2 in a 285/70/17 flavor (33x11.5) but it is mucho $$$.

Any and all opinions are welcome.