255/35R21 Tyre Size: Dimensions, Alternatives & Speedometer Effect

If you are looking up the 255/35R21 tyre size, the figures that matter are the diameter (711.9 mm, or 28.0 inches), the sidewall (89.2 mm), the circumference (2236.5 mm) and the revolutions per mile (719.6). The size code itself tells you the ingredients — 255 mm of width, an aspect ratio of 35%, a 21-inch rim — and this page shows how those ingredients combine into the measurements above, which sizes sit closest to it, and what happens to your speedometer if you fit one of them instead. Every figure here is computed from the size code, not estimated, so you can use it to check a tyre you already own or one you are thinking of buying.

Before the numbers, one practical point. A tyre size is only correct for your car if it matches what the manufacturer specified, and the place to find that is the placard on the driver's door jamb, not the size currently fitted — a previous owner may have changed it. The dimensions on this page tell you exactly what a 255/35R21 measures, so you can compare it against the placard figure and against any alternative you are considering. If the two do not match, the tables further down show how far apart they are and whether the difference stays inside the commonly used plus-or-minus 3% guideline.

This is a calculation, not a safety approval. Always check your vehicle's door-jamb placard, load index and speed rating, and confirm fitment with a tyre professional before fitting a different tyre size.

What does 255/35R21 mean?

The code breaks into three parts. The first number, 255, is the section width in millimetres — the width of the tyre from sidewall to sidewall when mounted on its measuring rim. The second number, 35, is the aspect ratio: the sidewall height expressed as a percentage of the width. Multiply them (255 x 35% = 89.2 mm) and you get the sidewall height. The final number, 21, is the rim diameter in inches — 21 x 25.4 = 533.4 mm. Add the rim diameter to twice the sidewall (533.4 + 2 x 89.2 = 711.9 mm) and you have the overall diameter. That is the whole formula, and it is why every size in the chart can be checked rather than trusted blindly.

255/35R21 dimensions

Measurement Value
Overall diameter 711.9 mm (28.0 in)
Sidewall height 89.2 mm
Circumference 2236.5 mm
Revolutions per mile 719.6
Rim diameter 21 in

Start with the circumference: 2236.5 mm per revolution. That is the distance your car covers every time this tyre turns once, and it is what makes the revs-per-mile figure (719.6) what it is — a mile divided by the circumference. Fit a tyre with a larger circumference and each revolution carries you further, so at the same wheel speed the car is moving faster than the instruments report. The sidewall (89.2 mm) is the flexible part: it absorbs the first shock from bumps and potholes, and a low-profile sidewall like this one means a short, stiff sidewall that sharpens steering response but leaves less cushioning over potholes and kerbs. These are nominal new-tyre dimensions, and real tyres vary slightly by brand, model, wear and inflation pressure.

Closest alternative sizes to 255/35R21

The three sizes below are the closest matches to 255/35R21 in the chart, drawn from the same rim or one inch either side and ranked by diameter difference. The closest, 255/40R20, differs by +0.0% (+0.1 mm). The commonly used guideline is to stay within plus or minus 3% of the original diameter, which keeps the speedometer error small and the gearing close to standard. That guideline is about the speedometer and the car's calibration, not about whether the tyre physically clears the arches — a size can be within 3% on diameter and still rub if it is much wider, so width and clearance need their own check.

Alternative size Diameter Sidewall Diameter difference Within ±3% guideline?
255/40R20 712.0 mm (28.0 in) 102.0 mm +0.0% Within the guideline
245/35R21 704.9 mm (27.8 in) 85.8 mm -1.0% Within the guideline
265/35R21 718.9 mm (28.3 in) 92.8 mm +1.0% Within the guideline

Speedometer effect of changing from 255/35R21

Take the closest alternative, 255/40R20, as a worked example. Its diameter is 712.0 mm against 711.9 mm here, a difference of +0.0%. Because the speedometer counts revolutions, the error follows the diameter ratio: actual speed equals indicated speed multiplied by 712.0 / 711.9. At an indicated 30 mph the car would really be doing 30.0 mph; at 50 mph indicated it would be 50.0 mph; at 70 mph indicated, 70.0 mph. That means the speedometer would read slightly high, so you would be travelling a touch slower than the dial suggests. Even a small percentage matters at motorway speed, which is why the plus-or-minus 3% guideline exists.

Replacement size Diameter change Actual speed at 30 mph indicated Actual speed at 50 mph indicated Actual speed at 70 mph indicated
255/40R20 +0.0% 30.0 mph 50.0 mph 70.0 mph
245/35R21 -1.0% 29.7 mph 49.5 mph 69.3 mph
265/35R21 +1.0% 30.3 mph 50.5 mph 70.7 mph

Wider, narrower or taller than 255/35R21: what actually changes

What actually changes if you move away from 255/35R21? Width first: a wider tyre puts more rubber on the road, which usually helps dry grip and can hurt fuel use a little through extra rolling resistance. Height second: because the sidewall is 35% of the width, widening without lowering the aspect ratio raises the diameter, lengthens the gearing and lifts the car slightly. A shorter sidewall sharpens steering response but transmits more of the road surface into the cabin. The practical rule is to hold the diameter as steady as you can — that is what the alternative table above is for — and to check that a wider tyre still clears the suspension and arches at full lock before committing.

How to check 255/35R21 fits your vehicle

The checks are the same whatever size you are considering. First, the door-jamb placard: size, load index and speed rating. Second, the rim: a 21-inch rim of the right width for a 255 mm tyre. Third, clearance — arches, suspension components and steering lock, with the car loaded as it will actually be used. Fourth, the pressure-monitoring system, which may need a reset or new sensors after a change. Fifth, the spare: if the spare is a different size, know its speed and distance limits before you need it. None of these is a formality, and together they are what turns a calculation into a safe fitment. For the 255/35R21, the 255 mm width and 89.2 mm sidewall are part of that same check.

Mistakes to avoid with 255/35R21

People go wrong in a handful of predictable ways. They trust the fitted tyre instead of the placard. They pick a wider tyre and forget the aspect ratio, so the diameter creeps up. They match the size code but buy a lower load index or speed rating. They mix sizes across an axle. They forget to check clearance at full steering lock, where a tyre that clears going straight can still rub. Each of these is easy to avoid once you know to look for it, and the tables on this page give you the numbers to check against. On a 255/35R21 the figures to check are a 711.9 mm diameter, a 89.2 mm sidewall and 719.6 revs per mile.

Related 255/35R21 sizes and tools

Frequently asked questions

What is the overall diameter of a 255/35R21 tyre?

The overall diameter of a new 255/35R21 tyre is 711.9 mm, which is 28.0 inches. That is the rim diameter (533.4 mm) plus twice the sidewall height (89.2 mm). It is a nominal figure, so an individual tyre can vary slightly by brand, model, wear and inflation pressure.

How tall is the sidewall on a 255/35R21 tyre?

For 255/35r21 tyre size, the point is this: The sidewall is 89.2 mm tall, because the aspect ratio is 35% of the 255 mm width (255 x 35/100 = 89.2 mm). A low-profile sidewall like this one means a short, stiff sidewall that sharpens steering response but leaves less cushioning over potholes and kerbs. The sidewall height is the main thing that decides how much cushioning the tyre gives over bumps.

How many revolutions per mile does a 255/35R21 tyre do?

A 255/35R21 tyre completes 719.6 revolutions per mile, based on its 2236.5 mm circumference. That figure matters because the speedometer and odometer count revolutions: fit a tyre with a different circumference and the readings drift by the same percentage as the diameter change.

What is the closest alternative size to 255/35R21?

The closest size in the chart is 255/40R20, which differs in diameter by +0.0% (+0.1 mm). The commonly used guideline is to stay within plus or minus 3% of the original diameter, and the alternative table on this page shows which sizes meet that. Diameter is only one check, though — load index, speed rating and physical clearance still need confirming.

Will changing from 255/35R21 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 255/40R20 instead of 255/35R21 changes the diameter by +0.0%, so an indicated 70 mph would really be 70.0 mph. Keep the diameter within plus or minus 3% to keep that error small, and check the placard before fitting anything different.