255/55R20 Tire Size: Dimensions, Alternatives & Speedometer Effect

Measured from the ground to the top of the tread, a new 255/55R20 tire stands 788.5 mm tall (31.0 inches). The sidewall — the rubber between the rim edge and the tread — is 140.2 mm high, the tire covers 2477.1 mm of road per revolution, and it completes 649.7 revolutions in a mile. Those numbers come straight from the three figures in the size code: 255 mm width, 55% aspect ratio and a 20-inch rim. This guide explains the maths behind them, lists the sizes that match this one most closely, and works out what fitting a different size would do to your speedometer reading.

Before the numbers, one practical point. A tire 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 255/55R20 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 255/55R20 mean?

Read the code left to right. 255 means the tire is 255 mm wide across the tread. 55 is the aspect ratio, so the sidewall is 55% of the width — 255 x 0.55 = 140.2 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 + 140.2 mm + 140.2 mm = 788.5 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.

255/55R20 dimensions

Measurement Value
Overall diameter 788.5 mm (31.0 in)
Sidewall height 140.2 mm
Circumference 2477.1 mm
Revolutions per mile 649.7
Rim diameter 20 in

The diameter of 788.5 mm (31.0 in) is the headline number because everything else follows from it. The circumference, 2477.1 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 649.7 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 255/55R20

Diameter is the right way to compare candidate replacements, and the table ranks the nearest three. The best match is 255/60R19 at +0.0% (+0.1 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?
255/60R19 788.6 mm (31.0 in) 153.0 mm +0.0% Within the guideline
275/55R19 785.1 mm (30.9 in) 151.2 mm -0.4% Within the guideline
285/50R20 793.0 mm (31.2 in) 142.5 mm +0.6% Within the guideline

Speedometer effect of changing from 255/55R20

Here is what the diameter difference does on the road. The speedometer assumes the original circumference, so a +0.0% diameter change from fitting 255/60R19 shifts every reading by the same percentage. Indicated 30 mph becomes 30.0 mph in reality, indicated 50 mph becomes 50.0 mph, and indicated 70 mph becomes 70.0 mph. The odometer drifts by the same percentage too, which also affects recorded mileage over time. If the difference were larger than 3%, recalibration would be worth discussing with a specialist rather than ignored.

Replacement size Diameter change Actual speed at 30 mph indicated Actual speed at 50 mph indicated Actual speed at 70 mph indicated
255/60R19 +0.0% 30.0 mph 50.0 mph 70.0 mph
275/55R19 -0.4% 29.9 mph 49.8 mph 69.7 mph
285/50R20 +0.6% 30.2 mph 50.3 mph 70.4 mph

Wider, narrower or taller than 255/55R20: what actually changes

Going wider than 255 mm while keeping the same aspect ratio also makes the tire 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 tire raises the car slightly and lengthens the gearing, so the engine turns a little slower at a given road speed; a shorter tire 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 255/55R20 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 255 mm tire. Then confirm physical clearance at full steering lock and under load, and confirm the pressure-monitoring system still works. A tire professional can run through all of that quickly, and that confirmation is the step this page cannot replace. The 255/55R20 measures 788.5 mm across, so that is the diameter the placard figure has to be compared with.

Mistakes to avoid with 255/55R20

Watch for these traps. Assuming the current tire 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 tire 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 255/55R20 is 255 mm wide on a 20-inch rim with a 788.5 mm overall diameter, so those are the numbers the placard check has to match.

Related 255/55R20 sizes and tools

Frequently asked questions

What is the overall diameter of a 255/55R20 tire?

The overall diameter of a new 255/55R20 tire is 788.5 mm, which is 31.0 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (140.2 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 255/55R20 tire?

The sidewall is 140.2 mm tall, because the aspect ratio is 55% of the 255 mm width (255 x 55/100 = 140.2 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 tire gives over bumps.

How many revolutions per mile does a 255/55R20 tire do?

A 255/55R20 tire completes 649.7 revolutions per mile, based on its 2477.1 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 255/55R20?

The closest size in the chart is 255/60R19, 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 255/55R20 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 255/60R19 instead of 255/55R20 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.