235/55R17 vs 235/50R18 — width held constant on purpose
A Mustang's factory plus-one, and the cleanest example of a manufacturer changing one variable at a time.
235/50R18 is 0.07 in taller than 235/55R17 (0.28%) and 0.00 in wider. A speedometer calibrated for 235/55R17 would under-read by 0.28%.
Measured against each other
Overall diameter
27.18 in27.25 in+0.07 in (+0.28%)Section width
9.25 in / 235 mm9.25 in / 235 mm0.00 in (0.00%)Sidewall height
5.09 in4.63 in-0.46 inCircumference
85.4 in85.6 in+0.2 inRevolutions per mile
742.1740.1-2.0Ride height at the axle
——+0.04 inSpeedometer reading
——+0.28% under-read
| Dimension | 235/55R17 | 235/50R18 | Change |
|---|---|---|---|
| Overall diameter | 27.18 in | 27.25 in | +0.07 in (+0.28%) |
| Section width | 9.25 in / 235 mm | 9.25 in / 235 mm | 0.00 in (0.00%) |
| Sidewall height | 5.09 in | 4.63 in | -0.46 in |
| Circumference | 85.4 in | 85.6 in | +0.2 in |
| Revolutions per mile | 742.1 | 740.1 | -2.0 |
| Ride height at the axle | — | — | +0.04 in |
| Speedometer reading | — | — | +0.28% under-read |
Calibrated for 235/55R17, running 235/50R18
- Dial reads 30
- 30.1 mph
- Dial reads 50
- 50.1 mph
- Dial reads 70
- 70.2 mph
- Odometer counts 100
- 100.3 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
Most plus-one pairs move width and aspect ratio together. This one does not touch the width at all.
One variable, one inch of wheel
Section width is 9.25 in on both sizes — the same casing, the same number of millimetres, no change whatsoever. The only thing that moves is the aspect ratio, from 55 to 50, and the sidewall falls from 5.09 in to 4.63 in.
Five points of a 235 mm casing releases 0.46 in per side, and the extra inch of wheel needs 0.50 in. The shortfall is four hundredths of an inch per side, so the diameter ends up 0.07 in taller: 27.18 in against 27.25 in.
Why holding the width matters here
Because on a rear-drive performance car the tire's footprint and the vehicle's suspension geometry were developed together, and changing the section width changes the scrub radius and the loads through the steering. Offering two wheel diameters with an identical casing width lets the manufacturer sell a wheel option without revisiting any of that.
It also makes the plus-one arithmetic easier to hit, since only one variable has to be balanced rather than two.
What the driver gets
A 0.46 in shorter sidewall and an inch more wheel. Speedometer error is 0.28%, and revolutions per mile moves from 742.1 to 740.1 — two rotations in a mile, out of seven hundred and forty. There is nothing to allow for.
Where the real difference sits
In stiffness and in replacement cost. Half an inch of sidewall is a large proportion of five inches of it, and an 18-inch wheel is more expensive to replace after a kerb strike than a 17. Those are the actual terms of the trade, and neither of them shows up in a table of diameters.
The rim underneath is not the same
Holding the section width does not mean the two wheels are the same. Both codes state 235 mm and both compute to 9.25 in, but that figure is quoted on a reference rim, and a wheel's width is not in a size code and is not in this site's data. If the two wheels differ in width — which nothing here can establish either way — the two tires will not measure the same across even though the codes are identical in that field, at something like 0.2 in per half inch of difference. It is the one place this pairing's tidy arithmetic can quietly understate the change.
The other Mustang question
Rear-tire width on a fixed wheel is the separate one: 255/40R19 against 275/35R19 changes the casing width by 20 mm on the same rim, which is the mirror image of this page — one variable again, but the other one.