255/50R20 Tyre Size: Dimensions, Alternatives & Speedometer Effect
The 255/50R20 is a medium-profile tyre size: 255 mm wide with a 50% aspect ratio on a 20-inch wheel. Its overall diameter is 763.0 mm (30.0 in), its sidewall stands 127.5 mm tall, it rolls 2397.0 mm per revolution and it turns 671.4 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.
This is a measurement page, not a recommendation. Whether a 255/50R20 belongs on your car depends on the placard figure, the load index and the speed rating your vehicle requires — none of which a diameter table can settle on its own. What the table can do is give you the exact size of this tyre (763.0 mm diameter, 127.5 mm sidewall, 2397.0 mm circumference) so that any comparison you make is grounded in real numbers. Work through the alternative-size table and the speedometer section before deciding anything, and treat the plus-or-minus 3% guideline as what it is: a rule of thumb, not an approval.
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 255/50R20 mean?
There are only three numbers to decode. Width: 255 mm. Aspect ratio: 50%, which fixes the sidewall at 255 x 50/100 = 127.5 mm. Rim: 20 inches, or 508.0 mm. The diameter is the rim diameter plus the sidewall top and bottom, giving 763.0 mm in total. Because the aspect ratio is a percentage of the width, a wider tyre with the same aspect ratio is also a taller tyre — a point that catches people out when they go wider without dropping the aspect ratio to compensate.
255/50R20 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 763.0 mm (30.0 in) |
| Sidewall height | 127.5 mm |
| Circumference | 2397.0 mm |
| Revolutions per mile | 671.4 |
| Rim diameter | 20 in |
The revs-per-mile figure, 671.4, 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 2397.0 mm circumference, this tyre turns 671.4 times per mile, and any replacement with a different circumference will turn a different number of times over the same mile. The diameter (763.0 mm, 30.0 in) and the sidewall (127.5 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 255/50R20
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. 255/55R19 is the nearest at +0.0% (+0.1 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? |
|---|---|---|---|---|
| 255/55R19 | 763.1 mm (30.0 in) | 140.2 mm | +0.0% | Within the guideline |
| 285/45R20 | 764.5 mm (30.1 in) | 128.2 mm | +0.2% | Within the guideline |
| 235/55R20 | 766.5 mm (30.2 in) | 129.2 mm | +0.5% | Within the guideline |
Speedometer effect of changing from 255/50R20
The speedometer effect is a straight ratio. Fitting 255/55R19 (763.1 mm) in place of 255/50R20 (763.0 mm) changes the diameter by +0.0%, so every indicated speed is multiplied by 1.000 to get the real speed. At 30 mph on the dial the true speed is 30.0 mph, at 50 mph it is 50.0 mph, and at 70 mph it is 70.0 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 |
|---|---|---|---|---|
| 255/55R19 | +0.0% | 30.0 mph | 50.0 mph | 70.0 mph |
| 285/45R20 | +0.2% | 30.1 mph | 50.1 mph | 70.1 mph |
| 235/55R20 | +0.5% | 30.1 mph | 50.2 mph | 70.3 mph |
Wider, narrower or taller than 255/50R20: what actually changes
Taking 255/50r20 tyre size as the case in point: Going wider than 255 mm while keeping the same aspect ratio also makes the tyre 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 tyre raises the car slightly and lengthens the gearing, so the engine turns a little slower at a given road speed; a shorter tyre 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 255/50R20 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 255 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 255/50R20, the 255 mm width and 127.5 mm sidewall are part of that same check.
Mistakes to avoid with 255/50R20
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 255/50R20 is 255 mm wide on a 20-inch rim with a 763.0 mm overall diameter, so those are the numbers the placard check has to match.
Related 255/50R20 sizes and tools
- 255/35R20 tyre size
- 255/40R20 tyre size
- Use the Tyre Size Calculator to compare 255/50R20 against any other size.
- Use the Plus Sizing Calculator to find a wider size that keeps the diameter close to 255/50R20.
Frequently asked questions
What is the overall diameter of a 255/50R20 tyre?
The overall diameter of a new 255/50R20 tyre is 763.0 mm, which is 30.0 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (127.5 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 255/50R20 tyre?
The sidewall is 127.5 mm tall, because the aspect ratio is 50% of the 255 mm width (255 x 50/100 = 127.5 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 255/50R20 tyre do?
A 255/50R20 tyre completes 671.4 revolutions per mile, based on its 2397.0 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 255/50R20?
The closest size in the chart is 255/55R19, 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 255/50R20 affect my speedometer?
Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 255/55R19 instead of 255/50R20 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.