265/55R20 Tyre Size: Dimensions, Alternatives & Speedometer Effect

Measured from the ground to the top of the tread, a new 265/55R20 tyre stands 799.5 mm tall (31.5 inches). The sidewall — the rubber between the rim edge and the tread — is 145.8 mm high, the tyre covers 2511.7 mm of road per revolution, and it completes 640.7 revolutions in a mile. Those numbers come straight from the three figures in the size code: 265 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 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 265/55R20 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 265/55R20 mean?

Read the code left to right. 265 means the tyre is 265 mm wide across the tread. 55 is the aspect ratio, so the sidewall is 55% of the width — 265 x 0.55 = 145.8 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 + 145.8 mm + 145.8 mm = 799.5 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.

265/55R20 dimensions

Measurement Value
Overall diameter 799.5 mm (31.5 in)
Sidewall height 145.8 mm
Circumference 2511.7 mm
Revolutions per mile 640.7
Rim diameter 20 in

The revs-per-mile figure, 640.7, 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 2511.7 mm circumference, this tyre turns 640.7 times per mile, and any replacement with a different circumference will turn a different number of times over the same mile. The diameter (799.5 mm, 31.5 in) and the sidewall (145.8 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 265/55R20

The three sizes below are the closest matches to 265/55R20 in the chart, drawn from the same rim or one inch either side and ranked by diameter difference. The closest, 265/60R19, differs by +0.1% (+1.1 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 +0.1% Within the guideline
285/50R20 793.0 mm (31.2 in) 142.5 mm -0.8% Within the guideline
255/60R19 788.6 mm (31.0 in) 153.0 mm -1.4% Within the guideline

Speedometer effect of changing from 265/55R20

Take the closest alternative, 265/60R19, as a worked example. Its diameter is 800.6 mm against 799.5 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 800.6 / 799.5. At an indicated 30 mph the car would really be doing 30.0 mph; at 50 mph indicated it would be 50.1 mph; at 70 mph indicated, 70.1 mph. That means the speedometer would read slightly low, so you would be travelling faster 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
265/60R19 +0.1% 30.0 mph 50.1 mph 70.1 mph
285/50R20 -0.8% 29.8 mph 49.6 mph 69.4 mph
255/60R19 -1.4% 29.6 mph 49.3 mph 69.0 mph

Wider, narrower or taller than 265/55R20: what actually changes

What actually changes if you move away from 265/55R20? 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 55% 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 265/55R20 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 265 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 265/55R20, the 265 mm width and 145.8 mm sidewall are part of that same check.

Mistakes to avoid with 265/55R20

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 265/55R20, that means checking the 265 mm width, the 55% aspect ratio and the 20-inch rim against the placard before you buy.

Related 265/55R20 sizes and tools

Frequently asked questions

What is the overall diameter of a 265/55R20 tyre?

The overall diameter of a new 265/55R20 tyre is 799.5 mm, which is 31.5 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (145.8 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 265/55R20 tyre?

The sidewall is 145.8 mm tall, because the aspect ratio is 55% of the 265 mm width (265 x 55/100 = 145.8 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 265/55R20 tyre do?

A 265/55R20 tyre completes 640.7 revolutions per mile, based on its 2511.7 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 265/55R20?

The closest size in the chart is 265/60R19, which differs in diameter by +0.1% (+1.1 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 265/55R20 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 265/55R20 changes the diameter by +0.1%, so an indicated 70 mph would really be 70.1 mph. Keep the diameter within plus or minus 3% to keep that error small, and check the placard before fitting anything different.