275/40R20 Tyre Size: Dimensions, Alternatives & Speedometer Effect
If you are looking up the 275/40R20 tyre size, the figures that matter are the diameter (728.0 mm, or 28.7 inches), the sidewall (110.0 mm), the circumference (2287.1 mm) and the revolutions per mile (703.7). The size code itself tells you the ingredients — 275 mm of width, an aspect ratio of 40%, a 20-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.
This is a measurement page, not a recommendation. Whether a 275/40R20 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 tyre (728.0 mm diameter, 110.0 mm sidewall, 2287.1 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 275/40R20 mean?
Read the code left to right. 275 means the tyre is 275 mm wide across the tread. 40 is the aspect ratio, so the sidewall is 40% of the width — 275 x 0.40 = 110.0 mm of rubber between rim and tread. R20 means it mounts on a 20-inch rim (508.0 mm). The overall diameter is the rim plus two sidewalls: 508.0 mm + 110.0 mm + 110.0 mm = 728.0 mm. Once you can do that sum, you can decode any size code you meet, and you can see why two sizes with the same rim can still be very different heights.
275/40R20 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 728.0 mm (28.7 in) |
| Sidewall height | 110.0 mm |
| Circumference | 2287.1 mm |
| Revolutions per mile | 703.7 |
| Rim diameter | 20 in |
The revs-per-mile figure, 703.7, 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 2287.1 mm circumference, this tyre turns 703.7 times per mile, and any replacement with a different circumference will turn a different number of times over the same mile. The diameter (728.0 mm, 28.7 in) and the sidewall (110.0 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 275/40R20
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 20 inches so the wheel itself stays realistic. 245/50R19 is the nearest at -0.1% (-0.4 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? |
|---|---|---|---|---|
| 245/50R19 | 727.6 mm (28.6 in) | 122.5 mm | -0.1% | Within the guideline |
| 245/45R20 | 728.5 mm (28.7 in) | 110.2 mm | +0.1% | Within the guideline |
| 225/55R19 | 730.1 mm (28.7 in) | 123.8 mm | +0.3% | Within the guideline |
Speedometer effect of changing from 275/40R20
The speedometer effect is a straight ratio. Fitting 245/50R19 (727.6 mm) in place of 275/40R20 (728.0 mm) changes the diameter by -0.1%, so every indicated speed is multiplied by 0.999 to get the real speed. At 30 mph on the dial the true speed is 30.0 mph, at 50 mph it is 50.0 mph, and at 70 mph it is 70.0 mph. The table below shows the same working for all three alternatives, so you can see at a glance which swap keeps the error negligible and which one starts to matter.
| Replacement size | Diameter change | Actual speed at 30 mph indicated | Actual speed at 50 mph indicated | Actual speed at 70 mph indicated |
|---|---|---|---|---|
| 245/50R19 | -0.1% | 30.0 mph | 50.0 mph | 70.0 mph |
| 245/45R20 | +0.1% | 30.0 mph | 50.0 mph | 70.0 mph |
| 225/55R19 | +0.3% | 30.1 mph | 50.1 mph | 70.2 mph |
Wider, narrower or taller than 275/40R20: what actually changes
What actually changes if you move away from 275/40R20? 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 40% 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 275/40R20 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 20-inch rim of the right width for a 275 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 275/40R20, the 275 mm width and 110.0 mm sidewall are part of that same check.
Mistakes to avoid with 275/40R20
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 275/40R20 the figures to check are a 728.0 mm diameter, a 110.0 mm sidewall and 703.7 revs per mile.
Related 275/40R20 sizes and tools
- 275/45R20 tyre size
- 275/50R20 tyre size
- Use the Tyre Size Calculator to compare 275/40R20 against any other size.
- Use the Plus Sizing Calculator to find a wider size that keeps the diameter close to 275/40R20.
Frequently asked questions
What is the overall diameter of a 275/40R20 tyre?
The overall diameter of a new 275/40R20 tyre is 728.0 mm, which is 28.7 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (110.0 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 275/40R20 tyre?
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 tyre gives over bumps. That is the point this page works through for 275/40r20 tyre size.
How many revolutions per mile does a 275/40R20 tyre do?
A 275/40R20 tyre completes 703.7 revolutions per mile, based on its 2287.1 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 275/40R20?
The closest size in the chart is 245/50R19, 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 275/40R20 affect my speedometer?
Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 245/50R19 instead of 275/40R20 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.