Something to ponder regarding tire size, odometer reading, and vehicle longevity

What got me thinking about these topics is that a friend of mine is considering selling his XJ. It is a 1998 SE model. The odometer reads 106k, and it has had 33" tires since 65k, and 30-31" before that. I have estimated that the actual distance traveled is closer to ~121k. With that said, the majority of the drivetrain has only rotated 106k worth of times.

What I am trying to figure out is the overall wear and tear on the vehicle. There are many things that affect this including the total amount of time the vehicle has been operated, total distance traveled in relation to rotations of the drivetrain, and stress applied to the various parts of the vehicle. For example, I would think that total time operated would have more of an effect on the engine than distance traveled; the vehicle can be at be at rest while the engine runs. Operation time is consistent across odometer readings though, meaning it shouldn't take you longer to get to your desination on 33s than it would on stock tires. With that said, this vehicle would still have 121k worth of operating time on it.

What about the rest of the drivetrain though? The axle shafts, etc have only made 106k worth of rotations, and I would think that the wear and tear on those parts would be less related to time operated and more related to distance traveled (rotations). It does take more force to rotate 33" tires in comparison to stock though, which could be translated into more stress on the drivetrain. So I guess the question is, does the added stress from larger tires cancel out the fact that there are less rotations of the drivetrain? Should i accept that the my friends truck has 121k, or should I think it has 106k and a longer engine operating time?

If the odometer is wrong, and you know it is, why care what the odometer says? Why think of it as being in comparable shape to the lower (inaccurate) mileage? Not that 15K on a Jeep means much, mechanically. It's a 10 year old vehicle with over 100K on it- it's going to need work at some point.

The other thing to consider is that the whole drive train has been working under more strain it's whole life rather than spinning more freely due to the larger tyres....

I would think of it as 140-150k miles. Bigger tires do not reduce the wear on anything, only increase it. Higher engine load, higher tranny load, higher axle bearing loads, higher transfer case load, etc., etc. Engine has been lugged more with higher connecting rod bearins loads, less oil flow, etc. XJ was designed and tested with stock tire sizes.

I don't think there is any way to know the actual wear put on parts with 33" vs stock tires, without doing experiments

Me thinks some people think things to death.

As soon as any modifications in gearing, tire size, weight of the vehicle, etc begin, it changes loads. As a couple of people have already implied, you need to generalize this a little bit. As with most parts, everything will show evidence of failure when it is thinking about dying. Just drive it and see what is making noise, seems out of shape, is leaking, and so on. Worrying about pseudo-mileage is not practical. And if the gearing of the axles has remained the same, then it is probably a little more worn than it may have been on 31s.

Interesting side note: My '00 came from the factory with the 225 size tires. when it got new tires last time through, I put 30s on it. With the 30s, the speedo/odo were exactly on. (checked against highway mile markers, time/distance/mileage, and most recently GPS. So, by running the factory size tires for the first 100K-odd miles, I've racked up an extra 2-3K on the clock.

Thanks for all the replies; the main reason I was asking was to decide if his asking price was legitimate given the condition of the jeep. I would be buying it as a daily driver and don't want it to be eating up my wallet in repair bills, but it can be difficult to foresee problems sometimes. Its a nice jeep- its got a RE 5.5 extreme duty short arm, bfg mud terrains, Chrysler 8.25 (not sure on spline count), HP Dana 35, new clutch last year. Needs a tune up and diff/tcase fluids changed soon and a few other minor repairs. He was initially asking 7500 now down to 6k. Id love to have it but not sure if its the best choice for a dd at this point. What do you guys think?

Does it have stock gears?, I dont see you saying if it did or not. I would think 6 is still even a little high for a 11 year old liffted jeep. It only blue books for around 3,000.

My 98 is on 33s with stock gears and i would go crazy if it was my dd, not just because of the gearing but because, the mud terranes whine, gets awful gas milage and always needs somting worked on. I did have it my dd when it had a 2inch lift and 30s and loved it.

If your looking for a DD i would recomend getting a stock jeep and doing a miled lift for the looks but still have the "nice things" of being pretty much stock, unless you weekend wheel it hard, then you will just end up going bigger with it!!

Honestly I think you're over-thinking this thing a bit. I would just do the normal things to determine the health of teh engine, such as a compression check or better yet a leakdown check. If the exterior of the vehicle is in good condition then it probably hasn't been beaten on much if at all. 106K is really not much mileage for a 4.0 that has been cared for properly.

tbburg said:

Interesting side note: My '00 came from the factory with the 225 size tires. when it got new tires last time through, I put 30s on it. With the 30s, the speedo/odo were exactly on. (checked against highway mile markers, time/distance/mileage, and most recently GPS. So, by running the factory size tires for the first 100K-odd miles, I've racked up an extra 2-3K on the clock.

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The odo should be correct with the stock tire size. It's independent from the speedo error.

winkosmosis said:

The odo should be correct with the stock tire size. It's independent from the speedo error.

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HUH?