255/35R20 Tyre Size: Dimensions, Alternatives & Speedometer Effect
If you are looking up the 255/35R20 tyre size, the figures that matter are the diameter (686.5 mm, or 27.0 inches), the sidewall (89.2 mm), the circumference (2156.7 mm) and the revolutions per mile (746.2). The size code itself tells you the ingredients — 255 mm of width, an aspect ratio of 35%, a 20-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 tyre you already own or one you are thinking of buying.
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 255/35R20 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 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 255/35R20 mean?
There are only three numbers to decode. Width: 255 mm. Aspect ratio: 35%, which fixes the sidewall at 255 x 35/100 = 89.2 mm. Rim: 20 inches, or 508.0 mm. The diameter is the rim diameter plus the sidewall top and bottom, giving 686.5 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.
255/35R20 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 686.5 mm (27.0 in) |
| Sidewall height | 89.2 mm |
| Circumference | 2156.7 mm |
| Revolutions per mile | 746.2 |
| Rim diameter | 20 in |
The diameter of 686.5 mm (27.0 in) is the headline number because everything else follows from it. The circumference, 2156.7 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 746.2 revolutions per mile — the figure the speedometer and odometer effectively count. A tyre with fewer revs per mile covers more ground per turn, so the car travels faster than the speedometer thinks if you fit a taller tyre without recalibrating. All of these are nominal new-tyre 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 tyre.
Closest alternative sizes to 255/35R20
Diameter is the right way to compare candidate replacements, and the table ranks the nearest three. The best match is 255/40R19 at +0.0% (+0.1 mm). Sizes inside plus or minus 3% of the original diameter are the ones usually considered for a swap, because the speedometer error they introduce is small. Outside that band the error grows and so do the knock-on effects on gearing, odometer readings and, on some cars, the calibration of driver-assistance systems that assume a standard rolling circumference.
| Alternative size | Diameter | Sidewall | Diameter difference | Within ±3% guideline? |
|---|---|---|---|---|
| 255/40R19 | 686.6 mm (27.0 in) | 102.0 mm | +0.0% | Within the guideline |
| 225/45R19 | 685.1 mm (27.0 in) | 101.2 mm | -0.2% | Within the guideline |
| 225/40R20 | 688.0 mm (27.1 in) | 90.0 mm | +0.2% | Within the guideline |
Speedometer effect of changing from 255/35R20
The speedometer effect is a straight ratio. Fitting 255/40R19 (686.6 mm) in place of 255/35R20 (686.5 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 |
|---|---|---|---|---|
| 255/40R19 | +0.0% | 30.0 mph | 50.0 mph | 70.0 mph |
| 225/45R19 | -0.2% | 29.9 mph | 49.9 mph | 69.9 mph |
| 225/40R20 | +0.2% | 30.1 mph | 50.1 mph | 70.2 mph |
Wider, narrower or taller than 255/35R20: what actually changes
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. That is the point this page works through for 255/35r20 tyre size.
How to check 255/35R20 fits your vehicle
Do the checks in this order. Read the placard and note the specified size, load index and speed rating for your exact vehicle. Compare the candidate size against the placard diameter using the plus-or-minus 3% guideline. Confirm the load index and speed rating are at least what the placard asks for. Confirm the rim width suits a 255 mm tyre. Then confirm physical clearance at full steering lock and under load, and confirm the pressure-monitoring system still works. A tyre professional can run through all of that quickly, and that confirmation is the step this page cannot replace. The 255/35R20 measures 686.5 mm across, so that is the diameter the placard figure has to be compared with.
Mistakes to avoid with 255/35R20
The most common mistake is reading the size off the tyre already fitted and assuming it is the correct size — a previous owner may have changed it. The second is going wider without adjusting the aspect ratio, which quietly makes the tyre taller and throws the speedometer off. The third is ignoring the load index and speed rating, which matter as much as the dimensions. The fourth is fitting different sizes on the same axle, which the car was not designed for. And the fifth is forgetting that the spare, the pressure-monitoring system and the speedometer all assume the standard size — change the size and all three need a thought. For a 255/35R20, that means checking the 255 mm width, the 35% aspect ratio and the 20-inch rim against the placard before you buy.
Related 255/35R20 sizes and tools
- 255/40R20 tyre size
- 255/45R20 tyre size
- Use the Tyre Size Calculator to compare 255/35R20 against any other size.
- Use the Plus Sizing Calculator to find a wider size that keeps the diameter close to 255/35R20.
Frequently asked questions
What is the overall diameter of a 255/35R20 tyre?
The overall diameter of a new 255/35R20 tyre is 686.5 mm, which is 27.0 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (89.2 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 255/35R20 tyre?
The sidewall is 89.2 mm tall, because the aspect ratio is 35% of the 255 mm width (255 x 35/100 = 89.2 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 tyre gives over bumps. That is the point this page works through for 255/35r20 tyre size.
How many revolutions per mile does a 255/35R20 tyre do?
A 255/35R20 tyre completes 746.2 revolutions per mile, based on its 2156.7 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 255/35R20?
The closest size in the chart is 255/40R19, 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 255/35R20 affect my speedometer?
Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 255/40R19 instead of 255/35R20 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.