Jeep Cherokee (XJ): small factory tires make every upgrade a big percentage

Three sizes, all on 15-inch wheels

215/75R15 measures 27.70 in, 225/75R15 measures 28.29 in, and 235/75R15 measures 28.88 in. All three are 75-series, all three sit on the platform's single wheel diameter, and each step of 10 mm in section width is worth 0.59 in of overall height.

That regularity is a consequence of holding the aspect ratio constant: the sidewall is 75% of whatever the width is, counted twice, so a wider tire is automatically a taller one by a fixed proportion. The full spread across the three is 1.18 in, or 4.26%.

Why a 33 is such a large step here

33x12.50R15 measures 33.00 in. From the middle factory size that is +4.71 in — a 16.66% increase — and from the narrowest, +5.30 in and 19.15%.

Put a 37-inch tire on a modern full-size truck starting at 32 inches and the proportional change is smaller than that. The XJ's small starting diameter is what makes its upgrade arithmetic so dramatic, and it is the reason the platform's suspension lift figures are quoted in inches that sound modest against the tire sizes they enable.

At 16.66%, an indicated 70 mph is really 81.7 mph, and the odometer records 100 miles for 116.7 actually driven. This is a platform where correcting the speedometer stops being optional bookkeeping and starts being the difference between a legal speed and a substantial one.

The intermediate sizes, which is where most XJs actually sit

• 30x9.50R15 at 30.00 in — +1.71 in from 225/75R15, 6.05%, and only 0.64 in wider.
• 31x10.50R15 at 31.00 in — +2.71 in, 9.59%, and 1.64 in wider.

The 31 is the classic fitment on this platform, and the arithmetic shows why: a step just under 10% with about an inch and a half more width, which is a modest change by the standards of what people do to these vehicles and a large one by the standards of anything else on this site.

An indicated 70 mph on 31s is 76.7.

Cable-driven instruments, and what that changes

This is an old enough platform that the speedometer on many examples is driven mechanically from the transmission rather than from wheel-speed sensors. That has two consequences worth knowing.

Recalibration is physical: a driven gear with a different tooth count, selected against the tire size and the axle ratio, rather than a value written by a scan tool. And because the signal is taken upstream of the axle, a change of axle ratio moves the speedometer as well as the tire size does — the two errors combine, which cuts both ways. Regear to put the engine back where it was after a tire change and the needle comes back with it, in one operation.

Speedometer calibration covers both architectures.

Eighteen model years, and no ratio worth printing for all of them

The ratio column is empty, and on a platform built across eighteen model years with several engines and two different rear axles that is close to unavoidable. It is also the field least likely to still hold on a surviving example: these are vehicles that have frequently been regeared by their owners, and the number on the tag is worth more than any model-level table.

Flotation notation is the native language here

All three of the commonly fitted sizes are stated in inches rather than millimetres, which is unusual on this site and entirely characteristic of the platform. These are sizes that came out of the off-road and agricultural tradition, where buyers wanted the height stated on the sidewall rather than derived from two other numbers.

The convention has a cost worth knowing: a flotation size states no aspect ratio, so nothing on a 31x10.50R15 tells you it is a 76-series tire. Comparing it with a metric size means converting first, and the size code page sets out how.

What the numbers do not settle

Whether any tire fits. These vehicles are old, heavily modified as a class, and their clearance depends entirely on what has been done to the individual example — suspension, fender trimming, wheel offset, steering components. A fender that has already been trimmed once, on a vehicle three owners ago, is not visible in any size code.