255/45R20 Tyre Size: Dimensions, Alternatives & Speedometer Effect
A 255/45R20 tyre is 255 mm wide, has an aspect ratio of 45% and fits a 20-inch rim. Worked through, that gives an overall diameter of 737.5 mm (29.0 inches), a sidewall height of 114.8 mm, a circumference of 2316.9 mm and 694.6 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 255/45R20 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 255/45R20 mean?
Read the code left to right. 255 means the tyre is 255 mm wide across the tread. 45 is the aspect ratio, so the sidewall is 45% of the width — 255 x 0.45 = 114.8 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 + 114.8 mm + 114.8 mm = 737.5 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.
255/45R20 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 737.5 mm (29.0 in) |
| Sidewall height | 114.8 mm |
| Circumference | 2316.9 mm |
| Revolutions per mile | 694.6 |
| Rim diameter | 20 in |
Start with the circumference: 2316.9 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 (694.6) 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 (114.8 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-tyre dimensions, and real tyres vary slightly by brand, model, wear and inflation pressure.
Closest alternative sizes to 255/45R20
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. 255/50R19 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? |
|---|---|---|---|---|
| 255/50R19 | 737.6 mm (29.0 in) | 127.5 mm | +0.0% | Within the guideline |
| 255/40R21 | 737.4 mm (29.0 in) | 102.0 mm | -0.0% | Within the guideline |
| 285/45R19 | 739.1 mm (29.1 in) | 128.2 mm | +0.2% | Within the guideline |
Speedometer effect of changing from 255/45R20
Take the closest alternative, 255/50R19, as a worked example. Its diameter is 737.6 mm against 737.5 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 737.6 / 737.5. 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 |
|---|---|---|---|---|
| 255/50R19 | +0.0% | 30.0 mph | 50.0 mph | 70.0 mph |
| 255/40R21 | -0.0% | 30.0 mph | 50.0 mph | 70.0 mph |
| 285/45R19 | +0.2% | 30.1 mph | 50.1 mph | 70.2 mph |
Wider, narrower or taller than 255/45R20: 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 tyre is mid-width, a middle-of-the-road width that balances grip, fuel use and road noise for everyday driving. Sidewall height drives comfort and response: at 114.8 mm this is a medium-profile tyre, 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 255/45R20 fits your vehicle
Start with the placard on the driver's door jamb: it lists the size, load index and speed rating the manufacturer specified for your vehicle, and it outranks whatever is currently fitted. Check that the load index of any replacement is at least the placard figure — a lower load index is not acceptable even if the size matches. Check the speed rating the same way, and check the rim width: a 255 mm tyre needs a rim within the width range the tyre maker approves for it. Then check clearance with the steering at full lock in both directions and the suspension compressed, and check the tyre-pressure monitoring system still reads correctly after the change. If any of those checks is unclear, a tyre professional can confirm fitment in minutes. The diameter being checked in this case is 737.5 mm for the 255/45R20.
Mistakes to avoid with 255/45R20
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 255/45R20 is 255 mm wide on a 20-inch rim with a 737.5 mm overall diameter, so those are the numbers the placard check has to match.
Related 255/45R20 sizes and tools
- 255/35R20 tyre size
- 255/40R20 tyre size
- Use the Tyre Size Calculator to compare 255/45R20 against any other size.
- Use the Plus Sizing Calculator to find a wider size that keeps the diameter close to 255/45R20.
Frequently asked questions
What is the overall diameter of a 255/45R20 tyre?
The overall diameter of a new 255/45R20 tyre is 737.5 mm, which is 29.0 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (114.8 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 255/45R20 tyre?
On this page, 255/45r20 tyre size is the case in point: The sidewall is 114.8 mm tall, because the aspect ratio is 45% of the 255 mm width (255 x 45/100 = 114.8 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 tyre gives over bumps.
How many revolutions per mile does a 255/45R20 tyre do?
A 255/45R20 tyre completes 694.6 revolutions per mile, based on its 2316.9 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 255/45R20?
The closest size in the chart is 255/50R19, 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 255/45R20 affect my speedometer?
Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 255/50R19 instead of 255/45R20 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.