285/50R20 Tyre Size: Dimensions, Alternatives & Speedometer Effect
The 285/50R20 is a medium-profile tyre size: 285 mm wide with a 50% aspect ratio on a 20-inch wheel. Its overall diameter is 793.0 mm (31.2 in), its sidewall stands 142.5 mm tall, it rolls 2491.3 mm per revolution and it turns 646.0 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 285/50R20 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 285/50R20 mean?
Read the code left to right. 285 means the tyre is 285 mm wide across the tread. 50 is the aspect ratio, so the sidewall is 50% of the width — 285 x 0.50 = 142.5 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 + 142.5 mm + 142.5 mm = 793.0 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.
285/50R20 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 793.0 mm (31.2 in) |
| Sidewall height | 142.5 mm |
| Circumference | 2491.3 mm |
| Revolutions per mile | 646.0 |
| Rim diameter | 20 in |
The revs-per-mile figure, 646.0, 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 2491.3 mm circumference, this tyre turns 646.0 times per mile, and any replacement with a different circumference will turn a different number of times over the same mile. The diameter (793.0 mm, 31.2 in) and the sidewall (142.5 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 285/50R20
The three sizes below are the closest matches to 285/50R20 in the chart, drawn from the same rim or one inch either side and ranked by diameter difference. The closest, 255/60R19, differs by -0.6% (-4.4 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? |
|---|---|---|---|---|
| 255/60R19 | 788.6 mm (31.0 in) | 153.0 mm | -0.6% | Within the guideline |
| 255/55R20 | 788.5 mm (31.0 in) | 140.2 mm | -0.6% | Within the guideline |
| 265/55R20 | 799.5 mm (31.5 in) | 145.8 mm | +0.8% | Within the guideline |
Speedometer effect of changing from 285/50R20
The speedometer effect is a straight ratio. Fitting 255/60R19 (788.6 mm) in place of 285/50R20 (793.0 mm) changes the diameter by -0.6%, so every indicated speed is multiplied by 0.994 to get the real speed. At 30 mph on the dial the true speed is 29.8 mph, at 50 mph it is 49.7 mph, and at 70 mph it is 69.6 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 |
|---|---|---|---|---|
| 255/60R19 | -0.6% | 29.8 mph | 49.7 mph | 69.6 mph |
| 255/55R20 | -0.6% | 29.8 mph | 49.7 mph | 69.6 mph |
| 265/55R20 | +0.8% | 30.2 mph | 50.4 mph | 70.6 mph |
Wider, narrower or taller than 285/50R20: what actually changes
What actually changes if you move away from 285/50R20? 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 50% 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 285/50R20 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 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 285/50R20, the 285 mm width and 142.5 mm sidewall are part of that same check.
Mistakes to avoid with 285/50R20
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 285/50R20 is 285 mm wide on a 20-inch rim with a 793.0 mm overall diameter, so those are the numbers the placard check has to match.
Related 285/50R20 sizes and tools
- 285/40R20 tyre size
- 285/45R20 tyre size
- Use the Tyre Size Calculator to compare 285/50R20 against any other size.
- Use the Plus Sizing Calculator to find a wider size that keeps the diameter close to 285/50R20.
Frequently asked questions
What is the overall diameter of a 285/50R20 tyre?
The overall diameter of a new 285/50R20 tyre is 793.0 mm, which is 31.2 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (142.5 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 285/50R20 tyre?
The sidewall is 142.5 mm tall, because the aspect ratio is 50% of the 285 mm width (285 x 50/100 = 142.5 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 285/50R20 tyre do?
A 285/50R20 tyre completes 646.0 revolutions per mile, based on its 2491.3 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 285/50R20?
The closest size in the chart is 255/60R19, which differs in diameter by -0.6% (-4.4 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 285/50R20 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 285/50R20 changes the diameter by -0.6%, so an indicated 70 mph would really be 69.6 mph. Keep the diameter within plus or minus 3% to keep that error small, and check the placard before fitting anything different.