255/40R20 Tire Size: Dimensions, Alternatives & Speedometer Effect
The 255/40R20 is a low-profile tire size: 255 mm wide with a 40% aspect ratio on a 20-inch wheel. Its overall diameter is 712.0 mm (28.0 in), its sidewall stands 102.0 mm tall, it rolls 2236.8 mm per revolution and it turns 719.5 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.
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 255/40R20 is a mid-width size, a short, stiff sidewall that sharpens steering response but leaves less cushioning over potholes and kerbs, 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 255/40R20 mean?
The code breaks into three parts. The first number, 255, is the section width in millimetres — the width of the tire from sidewall to sidewall when mounted on its measuring rim. The second number, 40, is the aspect ratio: the sidewall height expressed as a percentage of the width. Multiply them (255 x 40% = 102.0 mm) and you get the sidewall height. The final number, 20, is the rim diameter in inches — 20 x 25.4 = 508.0 mm. Add the rim diameter to twice the sidewall (508.0 + 2 x 102.0 = 712.0 mm) and you have the overall diameter. That is the whole formula, and it is why every size in the chart can be checked rather than trusted blindly.
255/40R20 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 712.0 mm (28.0 in) |
| Sidewall height | 102.0 mm |
| Circumference | 2236.8 mm |
| Revolutions per mile | 719.5 |
| Rim diameter | 20 in |
Start with the circumference: 2236.8 mm per revolution. That is the distance your car covers every time this tire turns once, and it is what makes the revs-per-mile figure (719.5) what it is — a mile divided by the circumference. Fit a tire 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 (102.0 mm) is the flexible part: it absorbs the first shock from bumps and potholes, and a low-profile sidewall like this one means a short, stiff sidewall that sharpens steering response but leaves less cushioning over potholes and kerbs. These are nominal new-tire dimensions, and real tyres vary slightly by brand, model, wear and inflation pressure.
Closest alternative sizes to 255/40R20
The three sizes below are the closest matches to 255/40R20 in the chart, drawn from the same rim or one inch either side and ranked by diameter difference. The closest, 255/45R19, differs by +0.0% (+0.1 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 tire 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? |
|---|---|---|---|---|
| 255/45R19 | 712.1 mm (28.0 in) | 114.8 mm | +0.0% | Within the guideline |
| 255/35R21 | 711.9 mm (28.0 in) | 89.2 mm | -0.0% | Within the guideline |
| 285/40R19 | 710.6 mm (28.0 in) | 114.0 mm | -0.2% | Within the guideline |
Speedometer effect of changing from 255/40R20
The speedometer effect is a straight ratio. Fitting 255/45R19 (712.1 mm) in place of 255/40R20 (712.0 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 |
|---|---|---|---|---|
| 255/45R19 | +0.0% | 30.0 mph | 50.0 mph | 70.0 mph |
| 255/35R21 | -0.0% | 30.0 mph | 50.0 mph | 70.0 mph |
| 285/40R19 | -0.2% | 29.9 mph | 49.9 mph | 69.9 mph |
Wider, narrower or taller than 255/40R20: 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 tire 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 102.0 mm this is a low-profile tire, a short, stiff sidewall that sharpens steering response but leaves less cushioning over potholes and kerbs. 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 255/40R20 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 tire needs a rim within the width range the tire 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 tire professional can confirm fitment in minutes. The diameter being checked in this case is 712.0 mm for the 255/40R20.
Mistakes to avoid with 255/40R20
Watch for these traps. Assuming the current tire 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 tire 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 255/40R20 is 255 mm wide on a 20-inch rim with a 712.0 mm overall diameter, so those are the numbers the placard check has to match.
Related 255/40R20 sizes and tools
- 255/35R20 tire size
- 255/45R20 tire size
- Use the Tire Size Calculator to compare 255/40R20 against any other size.
- Use the Plus Sizing Calculator to find a wider size that keeps the diameter close to 255/40R20.
Frequently asked questions
What is the overall diameter of a 255/40R20 tire?
The overall diameter of a new 255/40R20 tire is 712.0 mm, which is 28.0 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (102.0 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 255/40R20 tire?
The sidewall is 102.0 mm tall, because the aspect ratio is 40% of the 255 mm width (255 x 40/100 = 102.0 mm). A low-profile sidewall like this one means a short, stiff sidewall that sharpens steering response but leaves less cushioning over potholes and kerbs. The sidewall height is the main thing that decides how much cushioning the tire gives over bumps.
How many revolutions per mile does a 255/40R20 tire do?
A 255/40R20 tire completes 719.5 revolutions per mile, based on its 2236.8 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 255/40R20?
The closest size in the chart is 255/45R19, 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 255/40R20 affect my speedometer?
Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 255/45R19 instead of 255/40R20 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.