255/50R19 Tire Size: Dimensions, Alternatives & Speedometer Effect
If you are looking up the 255/50R19 tire size, the figures that matter are the diameter (737.6 mm, or 29.0 inches), the sidewall (127.5 mm), the circumference (2317.2 mm) and the revolutions per mile (694.5). The size code itself tells you the ingredients — 255 mm of width, an aspect ratio of 50%, a 19-inch rim — and this page shows how those ingredients combine into the measurements above, which sizes sit closest to it, and what happens to your speedometer if you fit one of them instead. Every figure here is computed from the size code, not estimated, so you can use it to check a tire you already own or one you are thinking of buying.
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 255/50R19 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 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 255/50R19 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: 19 inches, or 482.6 mm. The diameter is the rim diameter plus the sidewall top and bottom, giving 737.6 mm in total. Because the aspect ratio is a percentage of the width, a wider tire with the same aspect ratio is also a taller tire — a point that catches people out when they go wider without dropping the aspect ratio to compensate.
255/50R19 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 737.6 mm (29.0 in) |
| Sidewall height | 127.5 mm |
| Circumference | 2317.2 mm |
| Revolutions per mile | 694.5 |
| Rim diameter | 19 in |
The diameter of 737.6 mm (29.0 in) is the headline number because everything else follows from it. The circumference, 2317.2 mm, is pi times the diameter, and it is the distance the car travels each time the wheel turns once. Divide a mile (63,360 inches) by the circumference in inches and you get 694.5 revolutions per mile — the figure the speedometer and odometer effectively count. A tire with fewer revs per mile covers more ground per turn, so the car travels faster than the speedometer thinks if you fit a taller tire without recalibrating. All of these are nominal new-tire figures: real tyres vary slightly by brand, model, wear and inflation pressure, so treat the decimals as precise maths on a nominal size, not a measurement of your individual tire.
Closest alternative sizes to 255/50R19
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 19 inches so the wheel itself stays realistic. 255/55R18 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? |
|---|---|---|---|---|
| 255/55R18 | 737.7 mm (29.0 in) | 140.2 mm | +0.0% | Within the guideline |
| 255/45R20 | 737.5 mm (29.0 in) | 114.8 mm | -0.0% | Within the guideline |
| 235/60R18 | 739.2 mm (29.1 in) | 141.0 mm | +0.2% | Within the guideline |
Speedometer effect of changing from 255/50R19
Take the closest alternative, 255/55R18, as a worked example. Its diameter is 737.7 mm against 737.6 mm here, a difference of +0.0%. Because the speedometer counts revolutions, the error follows the diameter ratio: actual speed equals indicated speed multiplied by 737.7 / 737.6. At an indicated 30 mph the car would really be doing 30.0 mph; at 50 mph indicated it would be 50.0 mph; at 70 mph indicated, 70.0 mph. That means the speedometer would read slightly high, so you would be travelling a touch slower 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 |
|---|---|---|---|---|
| 255/55R18 | +0.0% | 30.0 mph | 50.0 mph | 70.0 mph |
| 255/45R20 | -0.0% | 30.0 mph | 50.0 mph | 70.0 mph |
| 235/60R18 | +0.2% | 30.1 mph | 50.1 mph | 70.2 mph |
Wider, narrower or taller than 255/50R19: what actually changes
What actually changes if you move away from 255/50R19? Width first: a wider tire 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 tire still clears the suspension and arches at full lock before committing.
How to check 255/50R19 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 19-inch rim of the right width for a 255 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 255/50R19, the 255 mm width and 127.5 mm sidewall are part of that same check.
Mistakes to avoid with 255/50R19
People go wrong in a handful of predictable ways. They trust the fitted tire instead of the placard. They pick a wider tire and forget the aspect ratio, so the diameter creeps up. They match the size code but buy a lower load index or speed rating. They mix sizes across an axle. They forget to check clearance at full steering lock, where a tire that clears going straight can still rub. Each of these is easy to avoid once you know to look for it, and the tables on this page give you the numbers to check against. On a 255/50R19 the figures to check are a 737.6 mm diameter, a 127.5 mm sidewall and 694.5 revs per mile.
Related 255/50R19 sizes and tools
- 255/35R19 tire size
- 255/40R19 tire size
- Use the Tire Size Calculator to compare 255/50R19 against any other size.
- Use the Plus Sizing Calculator to find a wider size that keeps the diameter close to 255/50R19.
Frequently asked questions
What is the overall diameter of a 255/50R19 tire?
The overall diameter of a new 255/50R19 tire is 737.6 mm, which is 29.0 inches. That is the rim diameter (482.6 mm) plus twice the sidewall height (127.5 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 255/50R19 tire?
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 tire gives over bumps.
How many revolutions per mile does a 255/50R19 tire do?
A 255/50R19 tire completes 694.5 revolutions per mile, based on its 2317.2 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 255/50R19?
The closest size in the chart is 255/55R18, 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 255/50R19 affect my speedometer?
Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 255/55R18 instead of 255/50R19 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.