255/40R21 Tyre Size: Dimensions, Alternatives & Speedometer Effect
The 255/40R21 is a low-profile tyre size: 255 mm wide with a 40% aspect ratio on a 21-inch wheel. Its overall diameter is 737.4 mm (29.0 in), its sidewall stands 102.0 mm tall, it rolls 2316.6 mm per revolution and it turns 694.7 times per mile. That last number is the one the speedometer cares about most, because the car counts wheel revolutions to work out speed. Below you will find the full measurement table, a plain-English decoding of the size code, the three closest alternative sizes from the chart, and a worked speedometer example for each of them.
People usually land on this page for one of three reasons: checking a tyre already on the car, shopping for a replacement, or wondering whether a slightly different size will work. In all three cases the approach is the same — start with the exact measurements, then compare. A 255/40R21 is a mid-width size, a short, stiff sidewall that sharpens steering response but leaves less cushioning over potholes and kerbs, and the tables below let you hold it up against the nearest alternatives on diameter, sidewall and revs per mile. The disclaimer box just below is part of that comparison: these are calculations, and the final fitment check belongs to your placard and a tyre professional.
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 255/40R21 mean?
There are only three numbers to decode. Width: 255 mm. Aspect ratio: 40%, which fixes the sidewall at 255 x 40/100 = 102.0 mm. Rim: 21 inches, or 533.4 mm. The diameter is the rim diameter plus the sidewall top and bottom, giving 737.4 mm in total. Because the aspect ratio is a percentage of the width, a wider tyre with the same aspect ratio is also a taller tyre — a point that catches people out when they go wider without dropping the aspect ratio to compensate.
255/40R21 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 737.4 mm (29.0 in) |
| Sidewall height | 102.0 mm |
| Circumference | 2316.6 mm |
| Revolutions per mile | 694.7 |
| Rim diameter | 21 in |
The revs-per-mile figure, 694.7, deserves attention because it is the number your car actually senses. Speedometers and odometers count wheel revolutions and multiply by an assumed circumference; change the circumference and the maths drifts. With a 2316.6 mm circumference, this tyre turns 694.7 times per mile, and any replacement with a different circumference will turn a different number of times over the same mile. The diameter (737.4 mm, 29.0 in) and the sidewall (102.0 mm) complete the picture. Remember these are nominal new-tyre figures — real tyres vary slightly by brand, model, wear and inflation pressure, so two tyres with the same size code can differ by a millimetre or two in practice.
Closest alternative sizes to 255/40R21
If you are looking for a substitute, start here. These are the three nearest sizes by overall diameter, limited to rims within one inch of 21 inches so the wheel itself stays realistic. 255/45R20 is the nearest at +0.0% (+0.1 mm difference). Anything within plus or minus 3% is generally treated as acceptable from a speedometer point of view, but diameter is only one of the checks: the replacement also needs the right load index and speed rating, and enough clearance around the suspension and arches at full steering lock.
| Alternative size | Diameter | Sidewall | Diameter difference | Within ±3% guideline? |
|---|---|---|---|---|
| 255/45R20 | 737.5 mm (29.0 in) | 114.8 mm | +0.0% | Within the guideline |
| 285/40R20 | 736.0 mm (29.0 in) | 114.0 mm | -0.2% | Within the guideline |
| 295/35R21 | 739.9 mm (29.1 in) | 103.2 mm | +0.3% | Within the guideline |
Speedometer effect of changing from 255/40R21
Take the closest alternative, 255/45R20, as a worked example. Its diameter is 737.5 mm against 737.4 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 737.5 / 737.4. 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 |
|---|---|---|---|---|
| 255/45R20 | +0.0% | 30.0 mph | 50.0 mph | 70.0 mph |
| 285/40R20 | -0.2% | 29.9 mph | 49.9 mph | 69.9 mph |
| 295/35R21 | +0.3% | 30.1 mph | 50.2 mph | 70.2 mph |
Wider, narrower or taller than 255/40R21: what actually changes
Taking 255/40r21 tyre size as the case in point: Three things move when the size moves. Diameter drives the speedometer and the gearing: taller means the engine revs slightly lower at cruising speed, shorter means it revs slightly higher. Width drives grip and rolling resistance: this tyre is mid-width, a middle-of-the-road width that balances grip, fuel use and road noise for everyday driving. Sidewall height drives comfort and response: at 102.0 mm this is a low-profile tyre, a short, stiff sidewall that sharpens steering response but leaves less cushioning over potholes and kerbs. Small changes in any one of these are usually barely noticeable; combined changes — much wider and much taller at once — are where the car's behaviour and the speedometer both start to shift in ways that need checking rather than assuming.
How to check 255/40R21 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 255/40R21, the 255 mm width and 102.0 mm sidewall are part of that same check.
Mistakes to avoid with 255/40R21
The most common mistake is reading the size off the tyre already fitted and assuming it is the correct size — a previous owner may have changed it. The second is going wider without adjusting the aspect ratio, which quietly makes the tyre taller and throws the speedometer off. The third is ignoring the load index and speed rating, which matter as much as the dimensions. The fourth is fitting different sizes on the same axle, which the car was not designed for. And the fifth is forgetting that the spare, the pressure-monitoring system and the speedometer all assume the standard size — change the size and all three need a thought. For a 255/40R21, that means checking the 255 mm width, the 40% aspect ratio and the 21-inch rim against the placard before you buy.
Related 255/40R21 sizes and tools
- 255/35R21 tyre size
- 245/35R21 tyre size
- Use the Tyre Size Calculator to compare 255/40R21 against any other size.
- Use the Plus Sizing Calculator to find a wider size that keeps the diameter close to 255/40R21.
Frequently asked questions
What is the overall diameter of a 255/40R21 tyre?
The overall diameter of a new 255/40R21 tyre is 737.4 mm, which is 29.0 inches. That is the rim diameter (533.4 mm) plus twice the sidewall height (102.0 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 255/40R21 tyre?
For 255/40r21 tyre size, the point is this: The sidewall is 102.0 mm tall, because the aspect ratio is 40% of the 255 mm width (255 x 40/100 = 102.0 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 255/40R21 tyre do?
A 255/40R21 tyre completes 694.7 revolutions per mile, based on its 2316.6 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 255/40R21?
The closest size in the chart is 255/45R20, 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 255/40R21 affect my speedometer?
Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 255/45R20 instead of 255/40R21 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.