Jeep Wrangler (TJ): ten-inch sidewall, fifteen-inch wheel

The factory sizes are small

215/75R15 at 27.70 in, 225/75R15 at 28.29 in and 245/75R16 at 30.47 in. The two 15-inch sizes are within 0.59 in of each other; the 16-inch fitment stands 2.77 in — 10.01% — above the smaller of them.

That is an enormous spread inside one model's factory range, and it means the phrase "stock TJ" is doing no work at all in a tire discussion.

What a 15-inch wheel buys

The sizes owners fit here are all flotation sizes on the factory 15-inch wheel, and the sidewall figures explain why the platform is so attached to them:

Size	Diameter	Sidewall
31x10.50R15	31.00 in	8.00 in
33x12.50R15	33.00 in	9.00 in
35x12.50R15	35.00 in	10.00 in

Ten inches of sidewall is more than any other tire recorded on this site, and it is arithmetic rather than design: a 35-inch tire on a 15-inch wheel has 20 inches of tire above and below the rim, and half of that is on each side.

Sidewall is what deflects over an obstacle, what allows very low pressures without unseating, and what keeps the rim away from rock. It is the reason 15-inch wheels persisted on this platform long after the rest of the industry moved to 17s and 18s.

The percentages are the largest here of anywhere

From 225/75R15:

• 31x10.50R15 — +2.71 in, 9.59%.
• 33x12.50R15 — +4.71 in, 16.66%. Indicated 70 mph is 81.7.
• 35x12.50R15 — +6.71 in, 23.73%. Indicated 70 mph is 86.6, and 100 miles of odometer is 123.7 actually covered.

From the narrower 215/75R15 the 35 is +7.30 in and 26.37%, which is the largest proportional change recorded across every platform on this site.

At that scale the speedometer stops being a curiosity. An indicated 60 mph on a 26% change is nearer 76, and no amount of familiarity with the vehicle compensates for an instrument that is wrong by a quarter.

The 16-inch trim starts somewhere else entirely

From 245/75R16, a 31x10.50R15 is a +0.53 in, 1.74% change — practically a substitution. The same tire from 215/75R15 is +3.30 in and 11.93%.

Two TJs, one tire size, and a factor of seven between the two changes. This is the platform where identifying the original size before doing any arithmetic matters most.

Gearing, and why nobody skips it here

The ratio column is empty, which on a vehicle of this age matters less than it would elsewhere: a surviving TJ is as likely to be carrying the ratio someone chose for it as the one it was built with. What the geometry says regardless: a 16.66% diameter increase behaves like a 16.66% numerically lower final drive, and a 23.73% increase like a 23.73% one.

Changes of that size take the vehicle well outside the range any factory ratio was chosen for, which is why regearing is discussed as a matter of course on this platform and as an afterthought on most others. The gear ratio calculator will take your own ratio and return what restores the original engine speed.

Speedometer correction on a vehicle this old

Many examples of this platform take their speed signal from the transmission rather than from the wheels, and on those the correction is mechanical: a driven gear with a different tooth count, chosen against both the tire diameter and the axle ratio.

That architecture has a useful property. Because the signal is picked up upstream of the ring and pinion, a regear moves the speedometer too — so a ratio change selected to restore engine speed after a large tire also brings the reading back toward true. The two corrections are the same ratio applied in opposite directions.

On later examples with wheel-speed sensors the two are independent, and the tire is the only term. Speedometer calibration sets out both cases.

Everything above is arithmetic

Not fitment. These vehicles are old, extensively modified as a class, and what clears depends on suspension, fender trimming, wheel offset and steering components on the specific example. The recorded upgrade sizes describe widespread practice, nothing more.