285/55R20 Tire Size: Dimensions, Alternatives & Speedometer Effect

The 285/55R20 is a medium-profile tire size: 285 mm wide with a 55% aspect ratio on a 20-inch wheel. Its overall diameter is 821.5 mm (32.3 in), its sidewall stands 156.8 mm tall, it rolls 2580.8 mm per revolution and it turns 623.6 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.

People usually land on this page for one of three reasons: checking a tire already on the car, shopping for a replacement, or wondering whether a slightly different size will work. In all three cases the approach is the same — start with the exact measurements, then compare. A 285/55R20 is a wide size, a balanced sidewall that splits the difference between handling response and ride comfort, and the tables below let you hold it up against the nearest alternatives on diameter, sidewall and revs per mile. The disclaimer box just below is part of that comparison: these are calculations, and the final fitment check belongs to your placard and a tire professional.

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 285/55R20 mean?

Read the code left to right. 285 means the tire is 285 mm wide across the tread. 55 is the aspect ratio, so the sidewall is 55% of the width — 285 x 0.55 = 156.8 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 + 156.8 mm + 156.8 mm = 821.5 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.

285/55R20 dimensions

Measurement Value
Overall diameter 821.5 mm (32.3 in)
Sidewall height 156.8 mm
Circumference 2580.8 mm
Revolutions per mile 623.6
Rim diameter 20 in

Start with the circumference: 2580.8 mm per revolution. That is the distance your car covers every time this tire turns once, and it is what makes the revs-per-mile figure (623.6) what it is — a mile divided by the circumference. Fit a tire 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 (156.8 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-tire dimensions, and real tyres vary slightly by brand, model, wear and inflation pressure.

Closest alternative sizes to 285/55R20

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. 275/55R20 is the nearest at -1.3% (-11.0 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?
275/55R20 810.5 mm (31.9 in) 151.2 mm -1.3% Within the guideline
275/60R20 838.0 mm (33.0 in) 165.0 mm +2.0% Within the guideline
265/60R19 800.6 mm (31.5 in) 159.0 mm -2.5% Within the guideline

Speedometer effect of changing from 285/55R20

The speedometer effect is a straight ratio. Fitting 275/55R20 (810.5 mm) in place of 285/55R20 (821.5 mm) changes the diameter by -1.3%, so every indicated speed is multiplied by 0.987 to get the real speed. At 30 mph on the dial the true speed is 29.6 mph, at 50 mph it is 49.3 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
275/55R20 -1.3% 29.6 mph 49.3 mph 69.1 mph
275/60R20 +2.0% 30.6 mph 51.0 mph 71.4 mph
265/60R19 -2.5% 29.2 mph 48.7 mph 68.2 mph

Wider, narrower or taller than 285/55R20: 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 156.8 mm this is a medium-profile tire, a balanced sidewall that splits the difference between handling response and ride comfort. 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 285/55R20 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 285 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 285/55R20, the 285 mm width and 156.8 mm sidewall are part of that same check.

Mistakes to avoid with 285/55R20

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 285/55R20, that means checking the 285 mm width, the 55% aspect ratio and the 20-inch rim against the placard before you buy.

Related 285/55R20 sizes and tools

Frequently asked questions

What is the overall diameter of a 285/55R20 tire?

The overall diameter of a new 285/55R20 tire is 821.5 mm, which is 32.3 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (156.8 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 285/55R20 tire?

The sidewall is 156.8 mm tall, because the aspect ratio is 55% of the 285 mm width (285 x 55/100 = 156.8 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 tire gives over bumps.

How many revolutions per mile does a 285/55R20 tire do?

A 285/55R20 tire completes 623.6 revolutions per mile, based on its 2580.8 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 285/55R20?

The closest size in the chart is 275/55R20, which differs in diameter by -1.3% (-11.0 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 285/55R20 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 275/55R20 instead of 285/55R20 changes the diameter by -1.3%, 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.