225/75R15 vs 31x10.50R15 — the size that defined a platform

Very few tires get associated with one vehicle. The 31x10.50R15 is one of them, and the XJ Cherokee is why.

What the factory fitted

225/75R15 on a 15x7 steel wheel: 28.29 in tall, 8.86 in wide, with a 6.64 in sidewall. It is an unremarkable specification and it was chosen for unremarkable reasons — it cleared without lifting anything, it was cheap, and it suited a vehicle sold mostly to people who drove it to work.

What the aftermarket fitted instead

The 31 is 31.00 in tall by definition, because in flotation notation the first number is the diameter. That is 2.71 in more, or 9.59%, and the sidewall grows from 6.64 in to 8.00 in with all of the change landing below and above a wheel that has not moved.

The width goes from 8.86 in to 10.50 in — 18.53% more casing. What that casing measures once it is on a wheel is a different question from what the code says, and the deciding number is the wheel's width, which is stamped on the wheel and is absent from both size codes and from this site. The narrower the wheel relative to the tire's reference rim, the more the tread edges are pulled in and the taller the tire stands relative to its nominal figure — which is one reason measured 31s on stock XJ steel wheels often come out closer to a true 31 than aggressive tread patterns usually manage.

Why 9.59% is the awkward number

It is too large to ignore and too small to force a decision. The engine turns about 9% slower at a given road speed, which on a low-revving straight-six is noticeable rather than crippling — and that is precisely why so many XJs ran 31s for years on the original 3.55 or 3.73 gears and their owners simply lived with it. The instrument error came along for the ride and got lived with too.

For reference: 3.55:1 in, 3.89:1 out, with 3.91:1 on the catalogue page. 3.73:1 in, 4.09:1 out, and 4.10:1 exists. Those are conversions of one number into another, nothing more.

The two dimensions behave differently on this vehicle

Height went up by 2.71 in and lifted the axle line by half that much. Width went up by 1.64 in and produced nothing measurable at all — until full lock, where it produces everything. The XJ's front track and its inner fender geometry are the constraint people actually meet, and they meet it steering, not driving straight.

That asymmetry is worth stating plainly because the two figures look comparable in a table and are not comparable in practice.

Where this pair sits in the ladder

Above it is the 33, which is another 2.00 in and a genuinely different proposition — the 31 against the 33 shows the second step is the same size as the first in inches and much larger in consequence. Below it is essentially nothing: 30x9.50R15 exists but was never common on this platform.

The aspect ratio nobody prints

The stock size announces its proportions: 75 means the sidewall is three-quarters of the section width. The 31 announces nothing of the kind, because flotation notation has no aspect-ratio field at all. Work it out and 31x10.50R15 comes to a 76-series tire — very slightly taller-profile than the metric size it replaced, which is not what most people would guess from a tire that looks so much more aggressive. The shape barely changed. The scale did.

The generational comparison

The Wrangler JK, two decades later, left the factory on 225/75R16 — a size that reaches 29.29 in on a wheel one inch larger, and lands much closer to the 31 than the XJ's 15-inch size ever did. The factory caught up with the aftermarket by moving the wheel, not the tire.

What is not on this page

Whether a 31 clears a particular XJ. Unit-body Jeeps vary by year, by trim and by whatever the previous owner did, and the answer depends on backspacing and bump-stop height rather than on either number above. The comparison tool will run any other pairing you want to test against the stock size.