275/40R21 Tire Size: Dimensions, Alternatives & Speedometer Effect

If you are looking up the 275/40R21 tire size, the figures that matter are the diameter (753.4 mm, or 29.7 inches), the sidewall (110.0 mm), the circumference (2366.9 mm) and the revolutions per mile (679.9). The size code itself tells you the ingredients — 275 mm of width, an aspect ratio of 40%, 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 tire you already own or one you are thinking of buying.

This is a measurement page, not a recommendation. Whether a 275/40R21 belongs on your car depends on the placard figure, the load index and the speed rating your vehicle requires — none of which a diameter table can settle on its own. What the table can do is give you the exact size of this tire (753.4 mm diameter, 110.0 mm sidewall, 2366.9 mm circumference) so that any comparison you make is grounded in real numbers. Work through the alternative-size table and the speedometer section before deciding anything, and treat the plus-or-minus 3% guideline as what it is: a rule of thumb, not an approval.

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 275/40R21 mean?

There are only three numbers to decode. Width: 275 mm. Aspect ratio: 40%, which fixes the sidewall at 275 x 40/100 = 110.0 mm. Rim: 21 inches, or 533.4 mm. The diameter is the rim diameter plus the sidewall top and bottom, giving 753.4 mm in total. Because the aspect ratio is a percentage of the width, a wider tire with the same aspect ratio is also a taller tire — a point that catches people out when they go wider without dropping the aspect ratio to compensate.

275/40R21 dimensions

Measurement Value
Overall diameter 753.4 mm (29.7 in)
Sidewall height 110.0 mm
Circumference 2366.9 mm
Revolutions per mile 679.9
Rim diameter 21 in

The diameter of 753.4 mm (29.7 in) is the headline number because everything else follows from it. The circumference, 2366.9 mm, is pi times the diameter, and it is the distance the car travels each time the wheel turns once. Divide a mile (63,360 inches) by the circumference in inches and you get 679.9 revolutions per mile — the figure the speedometer and odometer effectively count. A tire with fewer revs per mile covers more ground per turn, so the car travels faster than the speedometer thinks if you fit a taller tire without recalibrating. All of these are nominal new-tire figures: real tyres vary slightly by brand, model, wear and inflation pressure, so treat the decimals as precise maths on a nominal size, not a measurement of your individual tire.

Closest alternative sizes to 275/40R21

Diameter is the right way to compare candidate replacements, and the table ranks the nearest three. The best match is 245/50R20 at -0.1% (-0.4 mm). Sizes inside plus or minus 3% of the original diameter are the ones usually considered for a swap, because the speedometer error they introduce is small. Outside that band the error grows and so do the knock-on effects on gearing, odometer readings and, on some cars, the calibration of driver-assistance systems that assume a standard rolling circumference.

Alternative size Diameter Sidewall Diameter difference Within ±3% guideline?
245/50R20 753.0 mm (29.6 in) 122.5 mm -0.1% Within the guideline
275/45R20 755.5 mm (29.7 in) 123.8 mm +0.3% Within the guideline
285/35R22 758.3 mm (29.9 in) 99.8 mm +0.7% Within the guideline

Speedometer effect of changing from 275/40R21

Take the closest alternative, 245/50R20, as a worked example. Its diameter is 753.0 mm against 753.4 mm here, a difference of -0.1%. Because the speedometer counts revolutions, the error follows the diameter ratio: actual speed equals indicated speed multiplied by 753.0 / 753.4. 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
245/50R20 -0.1% 30.0 mph 50.0 mph 70.0 mph
275/45R20 +0.3% 30.1 mph 50.1 mph 70.2 mph
285/35R22 +0.7% 30.2 mph 50.3 mph 70.5 mph

Wider, narrower or taller than 275/40R21: what actually changes

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 tire is wide, a broad contact patch, which generally means more grip in the dry and more resistance to aquaplaning when the tread is in good condition. Sidewall height drives comfort and response: at 110.0 mm this is a low-profile tire, 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 275/40R21 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 275 mm tire. 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 275/40R21, the 275 mm width and 110.0 mm sidewall are part of that same check.

Mistakes to avoid with 275/40R21

The most common mistake is reading the size off the tire 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 tire 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 275/40R21, that means checking the 275 mm width, the 40% aspect ratio and the 21-inch rim against the placard before you buy.

Related 275/40R21 sizes and tools

Frequently asked questions

What is the overall diameter of a 275/40R21 tire?

The overall diameter of a new 275/40R21 tire is 753.4 mm, which is 29.7 inches. That is the rim diameter (533.4 mm) plus twice the sidewall height (110.0 mm). It is a nominal figure, so an individual tire can vary slightly by brand, model, wear and inflation pressure.

How tall is the sidewall on a 275/40R21 tire?

The sidewall is 110.0 mm tall, because the aspect ratio is 40% of the 275 mm width (275 x 40/100 = 110.0 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 tire gives over bumps.

How many revolutions per mile does a 275/40R21 tire do?

A 275/40R21 tire completes 679.9 revolutions per mile, based on its 2366.9 mm circumference. That figure matters because the speedometer and odometer count revolutions: fit a tire with a different circumference and the readings drift by the same percentage as the diameter change.

What is the closest alternative size to 275/40R21?

The closest size in the chart is 245/50R20, which differs in diameter by -0.1% (-0.4 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 275/40R21 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 245/50R20 instead of 275/40R21 changes the diameter by -0.1%, 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.