235/55R20 Tyre Size: Dimensions, Alternatives & Speedometer Effect

A 235/55R20 tyre is 235 mm wide, has an aspect ratio of 55% and fits a 20-inch rim. Worked through, that gives an overall diameter of 766.5 mm (30.2 inches), a sidewall height of 129.2 mm, a circumference of 2408.0 mm and 668.3 revolutions per mile. Those four numbers are the whole story of this size: they decide how tall the tyre stands, how far the car travels on each revolution, and how far the speedometer reading can drift if you change size. This page lays out the exact figures, explains what each one means in plain language, and shows the closest alternative sizes so you can judge a swap with numbers instead of guesswork.

People usually land on this page for one of three reasons: checking a tyre 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 235/55R20 is a mid-width 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 tyre 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 235/55R20 mean?

Read the code left to right. 235 means the tyre is 235 mm wide across the tread. 55 is the aspect ratio, so the sidewall is 55% of the width — 235 x 0.55 = 129.2 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 + 129.2 mm + 129.2 mm = 766.5 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.

235/55R20 dimensions

Measurement Value
Overall diameter 766.5 mm (30.2 in)
Sidewall height 129.2 mm
Circumference 2408.0 mm
Revolutions per mile 668.3
Rim diameter 20 in

Start with the circumference: 2408.0 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 (668.3) 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 (129.2 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 235/55R20

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. 285/45R20 is the nearest at -0.3% (-2.0 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?
285/45R20 764.5 mm (30.1 in) 128.2 mm -0.3% Within the guideline
255/55R19 763.1 mm (30.0 in) 140.2 mm -0.4% Within the guideline
255/50R20 763.0 mm (30.0 in) 127.5 mm -0.5% Within the guideline

Speedometer effect of changing from 235/55R20

Here is what the diameter difference does on the road. The speedometer assumes the original circumference, so a -0.3% diameter change from fitting 285/45R20 shifts every reading by the same percentage. Indicated 30 mph becomes 29.9 mph in reality, indicated 50 mph becomes 49.9 mph, and indicated 70 mph becomes 69.8 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
285/45R20 -0.3% 29.9 mph 49.9 mph 69.8 mph
255/55R19 -0.4% 29.9 mph 49.8 mph 69.7 mph
255/50R20 -0.5% 29.9 mph 49.8 mph 69.7 mph

Wider, narrower or taller than 235/55R20: what actually changes

What actually changes if you move away from 235/55R20? 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 235/55R20 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 235 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 235/55R20, the 235 mm width and 129.2 mm sidewall are part of that same check.

Mistakes to avoid with 235/55R20

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 235/55R20, that means checking the 235 mm width, the 55% aspect ratio and the 20-inch rim against the placard before you buy.

Related 235/55R20 sizes and tools

Frequently asked questions

What is the overall diameter of a 235/55R20 tyre?

The overall diameter of a new 235/55R20 tyre is 766.5 mm, which is 30.2 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (129.2 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 235/55R20 tyre?

The sidewall is 129.2 mm tall, because the aspect ratio is 55% of the 235 mm width (235 x 55/100 = 129.2 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 235/55R20 tyre do?

A 235/55R20 tyre completes 668.3 revolutions per mile, based on its 2408.0 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 235/55R20?

The closest size in the chart is 285/45R20, which differs in diameter by -0.3% (-2.0 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 235/55R20 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 285/45R20 instead of 235/55R20 changes the diameter by -0.3%, so an indicated 70 mph would really be 69.8 mph. Keep the diameter within plus or minus 3% to keep that error small, and check the placard before fitting anything different.