Our Jeep came with P225/70r15. Replaced with same size.

Last year we ruined a tire and had to switch to LT235/75r15 because they didn't have or couldn't get the 225/70. I haven't really checked the highway mileage since then but the last fill up was about 16 (lot's of short/slow driving). I don't know, it might be a little slower off the line?

It runs about 2200 rpm at 70 mph but the speedometer is probably off now. I try to cruise about 65 on the 2 lane.

To some extent, a larger diameter tire that's not too wide would improve highway gas mileage by lowering rpm.

 

hmm... do you recall where you saw that stat about not rolling under 1800 rpm in the 4.0? If that really is the case for a valid reason, then it would make sense for me to intentionally keep it at 40 or below in town to keep it out of OD. Like I said, mine seems to shift into OD when the speedo reads ~41~42mph. Now that's with the 235s so don't know exactly how far off it is with at the 40 range with just slightly larger tires than stock.
I've thought of going 31s next time too...but on my mild 2" lift, when I do go off road, I can get a lot more articulation and stuff the 235s without rubbing anything... I'm concerned that I'd be rubbing like it's going out of style on 31s with my 2" lift.
I want my XJ to last a lot longer...so I dont really want to stress it to the max of what it can take without breaking something. If 235s really do make a lot more sense, then I'll just get a better tire than what I currently have when it's time to replace. I just like the look of 31s on my lift...but I'm not so vain as to give up function...she's not a mall crawler.. she actually gets used. (not abused though)

 

I used a tire size calculator when I got my 235's and I think the difference was about 5 mph at 70, so that would be 3-4 mph in the 40-50 mph range.

Can't you just leave it in the middle gear to keep it out of OD? But I can't really believe the <1800 rpm thing.

My 235's rub the control arm on tight turns.

 

The only reason that comes to me of the top of my head is that the lower the rpm, the more likelihood there is to get things like increased carbon deposit buildup, etc.

I've never rubbed with my 235s on a 2" lift.... are you running a different wheel? maybe different backspacing?

 

Wish I could remember where I read about the 1800 rpm range (still looking!). I know it had to do with operating an auto equipped XJ at low speeds and the fluid temp. And as a side advantage to keeping the rpm's to at least 1800rpm by running in the 3 position for cooling purposes the higher vacuum gave better mpg. Remember now I was referring to my average operating speed in the 35 to 45mph range and not highway speeds.

My 235's on OEM steel rims (5.25" BS) would rub the lower control arms a bit in off camber positions only.

 

Interesting... well I guess I'll just keep driving her as I always have then, that way it keeps the rpms up a bit higher....would still love to see the doc if you find it. I'm in a enormous city now (still getting used to it)...and there are so many service streets that unless I get on one of the Interstates, loops, whatever, my max speed is generally under 50. I do like to get it up at highway speeds whenever I can just to run her at the higher rpm range etc. to clean it out more.
On the tires, like I said, I've never noticed rubbing with mine, but then, I'm not at full lock very often either. so maybe that's why...I don't recall seeing any rub marks on the LCAs...but I'll have to check closer now just to see. You got me curious.

 

I never had any rubbing when on level ground at full lock in either direction. Even going straight in a washout on just the right or left side they wouldn't. But if I were to run one that had a pretty tight turn in it they would rub. Nothing extreme ever.

Another reason I like running in 3rd is that the higher rpm's act like an engine brake. We have plenty of swerving roads with tight turns and many being on a steep declines. Match that up with the fact that the roads are NARROW and that hold back when letting off the throttle is nice.

 

Are you running a lift on yours?

 

Yea. 2" in the front and 2+ in the back. Not pretty but very functional for my needs so far.

 

That's what I have... 2" in the front and 2+ in the back with heavy duty leaf springs. along with new longer shocks all around. Nothing fancy, just functional for what I need... some pro comp 5000s
The originals were sad and completely shot. going with a mild lift, didn't want to leave orig. height shocks so went with some that were a little taller. My wife isn't super fond of my jeep... plus she's only 5' so I have to help her up into it. lol.

 

Well, I have no lift. It's actually a bit saggy in the rear.

The wheels are these weird fancy gold honeycomb things that I assume came stock on this "Limited" model. I don't much care for them but there isn't too much need to replace them with some plain steel ones.

I might think about a small lift after I get some other things fixed. But I don't want it too high to throw six gallon water jugs in there! Our 4runner was about killing me while our Jeep was in the shop.

 

"I don't want it too high to throw six gallon water jugs in there! Our 4runner was about killing me while our Jeep was in the shop." Smart!

I use two milk crates with four of those Arizona tea jugs in each to get our spring water. GLAD the XJ isn't any higher.

 

225/70r15 is 27.4” diameter, circumference is 86.04”. (My Jeep’s stock size is 215/75r15 which is 27.7” diameter, circumference is 86,97”))

235/75r15 is 28.9” diameter, circumference is 90.75”

I also run a 235/75r15. It definitely ate up some of my power / acceleration, but not drastically. The weight of the tire is probably eating up more fuel mileage than the diameter since it’s a 6-ply tire. Your LT rated tires are probably 6-ply rated at least, maybe heavier.

 

We have to haul hot water to our horse barn to thaw the ice in the water trough. The little jugs would cool off too fast and then I'd have to stand there pouring them in the trough one at a time instead of tipping the 6 gal jug(s) on the edge of the tank and balancing against a fence rail. The tank sits under the fence so it can be filled from the outside right next to where we back in the jeep. Just pull the jug out of the back of the jeep, turn around, and sit it on it's side on the edge of the tank (take off lid of course!). The horses drink from the other side of the fence and the deer drink from all sides. In the summer we have a surface line that runs over there to a bigger trough.

Out here on these roads a 6 ply tire barely holds up. Our old square body suburban would eat 6 ply tires like crazy and it was only 15" wheels so you couldn't get anything heavier, but on the little XJ they should hold up fairly well. Everything else we own now has 16 or 17 inch 10 ply tires.

What I would like to have on the jeep is some narrow steel 15" wheels and 700-15 bias tires. Sure they don't "last" very long compared to radials but much more resistant to rock damage. We never busted tires on our 71 K10 with camper that would have been about the same weight as the Suburban. The way we drive it, the jeep tires will rot before we ever get to 10-15 thousand miles.

 

I hear you on tires. I thought about going to 16” wheels so I could get a better tire choice, but it’s not all that much better. I ordered the tires I have on my Jeep because the only 235/75-15 in stock around here were 4 ply. 6 ply holds up pretty good on my XJ, but I run 10 ply on my F150. XJ’s are light enough to get by with 6 ply in most cases, but if I could find 235/75-15 in 10 ply, I’d run a set.