255/70R18 vs 265/70R18 — small enough for air suspension
The Defender's conservative option, and why a vehicle that adjusts its own ride height has less tolerance for tire changes rather than more.
265/70R18 is 0.55 in taller than 255/70R18 (1.72%) and 0.39 in wider. A speedometer calibrated for 255/70R18 would under-read by 1.72%.
Measured against each other
Overall diameter
32.06 in32.61 in+0.55 in (+1.72%)Section width
10.04 in / 255 mm10.43 in / 265 mm+0.39 in (+3.92%)Sidewall height
7.03 in7.30 in+0.28 inCircumference
100.7 in102.4 in+1.7 inRevolutions per mile
629.2618.5-10.6Ride height at the axle
——+0.28 inSpeedometer reading
——+1.72% under-read
| Dimension | 255/70R18 | 265/70R18 | Change |
|---|---|---|---|
| Overall diameter | 32.06 in | 32.61 in | +0.55 in (+1.72%) |
| Section width | 10.04 in / 255 mm | 10.43 in / 265 mm | +0.39 in (+3.92%) |
| Sidewall height | 7.03 in | 7.30 in | +0.28 in |
| Circumference | 100.7 in | 102.4 in | +1.7 in |
| Revolutions per mile | 629.2 | 618.5 | -10.6 |
| Ride height at the axle | — | — | +0.28 in |
| Speedometer reading | — | — | +1.72% under-read |
Calibrated for 255/70R18, running 265/70R18
- Dial reads 30
- 30.5 mph
- Dial reads 50
- 50.9 mph
- Dial reads 70
- 71.2 mph
- Odometer counts 100
- 101.7 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
A vehicle with air suspension looks like the one most able to absorb a tire change. It is generally the one least able to.
Why the platform constrains the choice
Air suspension does not float free of the tire. The system holds the body at a commanded height, measured by ride-height sensors, and every one of its positions — access, standard, off-road, and the wading modes above them — was calibrated with a specific rolling radius underneath. A taller tire raises the axle relative to the ground while the body stays where the sensors put it, so what changes is the clearance between the tire and the arch at every one of those positions, including the highest.
That is why the Defender's practical range of tire sizes is narrow, and why a 0.55 in step is the one usually discussed rather than the two-inch steps that a solid-axle truck would tolerate.
A quarter of an inch, and that is the whole of it
The axle line finishes about a quarter of an inch higher. On a vehicle that raises and lowers itself through several inches on command, the suspension will never notice a figure that small. What narrows is the space above the crown: the same quarter inch comes out of it in every mode the vehicle offers, and that space — not ride height — is where this platform's tolerance is genuinely narrow rather than merely small.
Both codes end in R18, so nothing here is a wheel-diameter change. The other wheel dimension —
width — is not in either code and is not held anywhere on this site, so what these two casings ask
of a particular set of Defender wheels is not something the arithmetic reaches. Ten millimetres of
nominal section is 0.39 in on paper, and half an inch of rim width shifts a measured section width
by something like 0.2 in on its own, which is the scale the two numbers should be read against. The
width of the wheel you have is stamped on it, second figure in a marking like 18x8 ET40.
What the arithmetic does settle is the instrument: 1.72% of speedometer error to carry or ignore, which is inside the band a single set of tires moves through between new and worn.
The other Defender comparison
The more interesting one is between wheel sizes rather than tire sizes: 255/65R19 against 255/70R18 puts the two factory wheel packages against each other and finds them identical in rolling diameter to a hundredth of an inch. Read together, the two pages describe the whole of this platform's available range — which is deliberately small.
What is left unmeasured
The load index the vehicle requires, which is printed on the door-jamb placard and is high on a vehicle of this weight, and whether the specific tire carries it. That is a property of a tire, not of a size code.