275/55R20 Tyre Size: Dimensions, Alternatives & Speedometer Effect
The 275/55R20 is a medium-profile tyre size: 275 mm wide with a 55% aspect ratio on a 20-inch wheel. Its overall diameter is 810.5 mm (31.9 in), its sidewall stands 151.2 mm tall, it rolls 2546.3 mm per revolution and it turns 632.0 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.
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 275/55R20 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 275/55R20 mean?
There are only three numbers to decode. Width: 275 mm. Aspect ratio: 55%, which fixes the sidewall at 275 x 55/100 = 151.2 mm. Rim: 20 inches, or 508.0 mm. The diameter is the rim diameter plus the sidewall top and bottom, giving 810.5 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.
275/55R20 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 810.5 mm (31.9 in) |
| Sidewall height | 151.2 mm |
| Circumference | 2546.3 mm |
| Revolutions per mile | 632.0 |
| Rim diameter | 20 in |
Start with the circumference: 2546.3 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 (632.0) 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 (151.2 mm) is the flexible part: it absorbs the first shock from bumps and potholes, and a medium-profile sidewall like this one means a balanced sidewall that splits the difference between handling response and ride comfort. These are nominal new-tyre dimensions, and real tyres vary slightly by brand, model, wear and inflation pressure.
Closest alternative sizes to 275/55R20
The three sizes below are the closest matches to 275/55R20 in the chart, drawn from the same rim or one inch either side and ranked by diameter difference. The closest, 265/60R19, differs by -1.2% (-9.9 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? |
|---|---|---|---|---|
| 265/60R19 | 800.6 mm (31.5 in) | 159.0 mm | -1.2% | Within the guideline |
| 265/55R20 | 799.5 mm (31.5 in) | 145.8 mm | -1.4% | Within the guideline |
| 285/55R20 | 821.5 mm (32.3 in) | 156.8 mm | +1.4% | Within the guideline |
Speedometer effect of changing from 275/55R20
The speedometer effect is a straight ratio. Fitting 265/60R19 (800.6 mm) in place of 275/55R20 (810.5 mm) changes the diameter by -1.2%, so every indicated speed is multiplied by 0.988 to get the real speed. At 30 mph on the dial the true speed is 29.6 mph, at 50 mph it is 49.4 mph, and at 70 mph it is 69.1 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 |
|---|---|---|---|---|
| 265/60R19 | -1.2% | 29.6 mph | 49.4 mph | 69.1 mph |
| 265/55R20 | -1.4% | 29.6 mph | 49.3 mph | 69.0 mph |
| 285/55R20 | +1.4% | 30.4 mph | 50.7 mph | 71.0 mph |
Wider, narrower or taller than 275/55R20: what actually changes
Taking 275/55r20 tyre size as the case in point: Going wider than 275 mm while keeping the same aspect ratio also makes the tyre taller, because the sidewall is a percentage of the width — that is the single most common surprise in a size change. Going wider and dropping the aspect ratio keeps the diameter closer to standard, which is the idea behind plus-sizing. A taller tyre raises the car slightly and lengthens the gearing, so the engine turns a little slower at a given road speed; a shorter tyre does the opposite. Wider rubber generally brings more dry grip and more road noise, and it can increase fuel use slightly through higher rolling resistance. None of these effects is dramatic for a small change, but they all point the same way: change one dimension at a time and keep the diameter close.
How to check 275/55R20 fits your vehicle
Do the checks in this order. Read the placard and note the specified size, load index and speed rating for your exact vehicle. Compare the candidate size against the placard diameter using the plus-or-minus 3% guideline. Confirm the load index and speed rating are at least what the placard asks for. Confirm the rim width suits a 275 mm tyre. Then confirm physical clearance at full steering lock and under load, and confirm the pressure-monitoring system still works. A tyre professional can run through all of that quickly, and that confirmation is the step this page cannot replace. The 275/55R20 measures 810.5 mm across, so that is the diameter the placard figure has to be compared with.
Mistakes to avoid with 275/55R20
Watch for these traps. Assuming the current tyre is the correct one, instead of checking the placard. Changing width without compensating the aspect ratio, so the diameter drifts. Buying on size alone and overlooking load index and speed rating. Mixing different sizes on the same axle. And skipping the clearance check at full lock, which is exactly where a wider tyre usually makes contact. If you avoid those five, and keep the diameter within plus or minus 3% of the placard size, you have dealt with the common errors before they happen. A 275/55R20 is 275 mm wide on a 20-inch rim with a 810.5 mm overall diameter, so those are the numbers the placard check has to match.
Related 275/55R20 sizes and tools
- 275/40R20 tyre size
- 275/45R20 tyre size
- Use the Tyre Size Calculator to compare 275/55R20 against any other size.
- Use the Plus Sizing Calculator to find a wider size that keeps the diameter close to 275/55R20.
Frequently asked questions
What is the overall diameter of a 275/55R20 tyre?
The overall diameter of a new 275/55R20 tyre is 810.5 mm, which is 31.9 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (151.2 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 275/55R20 tyre?
For 275/55r20 tyre size, the point is this: The sidewall is 151.2 mm tall, because the aspect ratio is 55% of the 275 mm width (275 x 55/100 = 151.2 mm). A medium-profile sidewall like this one means a balanced sidewall that splits the difference between handling response and ride comfort. The sidewall height is the main thing that decides how much cushioning the tyre gives over bumps.
How many revolutions per mile does a 275/55R20 tyre do?
A 275/55R20 tyre completes 632.0 revolutions per mile, based on its 2546.3 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 275/55R20?
The closest size in the chart is 265/60R19, which differs in diameter by -1.2% (-9.9 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 275/55R20 affect my speedometer?
Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 265/60R19 instead of 275/55R20 changes the diameter by -1.2%, so an indicated 70 mph would really be 69.1 mph. Keep the diameter within plus or minus 3% to keep that error small, and check the placard before fitting anything different.