255/40R19 Tyre Size: Dimensions, Alternatives & Speedometer Effect
If you are looking up the 255/40R19 tyre size, the figures that matter are the diameter (686.6 mm, or 27.0 inches), the sidewall (102.0 mm), the circumference (2157.0 mm) and the revolutions per mile (746.1). The size code itself tells you the ingredients — 255 mm of width, an aspect ratio of 40%, 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 tyre you already own or one you are thinking of buying.
Before the numbers, one practical point. A tyre size is only correct for your car if it matches what the manufacturer specified, and the place to find that is the placard on the driver's door jamb, not the size currently fitted — a previous owner may have changed it. The dimensions on this page tell you exactly what a 255/40R19 measures, so you can compare it against the placard figure and against any alternative you are considering. If the two do not match, the tables further down show how far apart they are and whether the difference stays inside the commonly used plus-or-minus 3% guideline.
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 255/40R19 mean?
The code breaks into three parts. The first number, 255, is the section width in millimetres — the width of the tyre 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, 19, is the rim diameter in inches — 19 x 25.4 = 482.6 mm. Add the rim diameter to twice the sidewall (482.6 + 2 x 102.0 = 686.6 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.
255/40R19 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 686.6 mm (27.0 in) |
| Sidewall height | 102.0 mm |
| Circumference | 2157.0 mm |
| Revolutions per mile | 746.1 |
| Rim diameter | 19 in |
Start with the circumference: 2157.0 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 (746.1) 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 (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-tyre dimensions, and real tyres vary slightly by brand, model, wear and inflation pressure.
Closest alternative sizes to 255/40R19
Diameter is the right way to compare candidate replacements, and the table ranks the nearest three. The best match is 255/45R18 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? |
|---|---|---|---|---|
| 255/45R18 | 686.7 mm (27.0 in) | 114.8 mm | +0.0% | Within the guideline |
| 255/35R20 | 686.5 mm (27.0 in) | 89.2 mm | -0.0% | Within the guideline |
| 225/45R19 | 685.1 mm (27.0 in) | 101.2 mm | -0.2% | Within the guideline |
Speedometer effect of changing from 255/40R19
Take the closest alternative, 255/45R18, as a worked example. Its diameter is 686.7 mm against 686.6 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 686.7 / 686.6. 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 |
|---|---|---|---|---|
| 255/45R18 | +0.0% | 30.0 mph | 50.0 mph | 70.0 mph |
| 255/35R20 | -0.0% | 30.0 mph | 50.0 mph | 70.0 mph |
| 225/45R19 | -0.2% | 29.9 mph | 49.9 mph | 69.8 mph |
Wider, narrower or taller than 255/40R19: what actually changes
For 255/40r19 tyre size, the point is this: 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 102.0 mm this is a low-profile tyre, 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 255/40R19 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 255/40R19 measures 686.6 mm across, so that is the diameter the placard figure has to be compared with.
Mistakes to avoid with 255/40R19
People go wrong in a handful of predictable ways. They trust the fitted tyre instead of the placard. They pick a wider tyre 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 tyre 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 255/40R19 the figures to check are a 686.6 mm diameter, a 102.0 mm sidewall and 746.1 revs per mile.
Related 255/40R19 sizes and tools
- 255/35R19 tyre size
- 255/45R19 tyre size
- Use the Tyre Size Calculator to compare 255/40R19 against any other size.
- Use the Plus Sizing Calculator to find a wider size that keeps the diameter close to 255/40R19.
Frequently asked questions
What is the overall diameter of a 255/40R19 tyre?
The overall diameter of a new 255/40R19 tyre is 686.6 mm, which is 27.0 inches. That is the rim diameter (482.6 mm) plus twice the sidewall height (102.0 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 255/40R19 tyre?
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 tyre gives over bumps. That is the point this page works through for 255/40r19 tyre size.
How many revolutions per mile does a 255/40R19 tyre do?
A 255/40R19 tyre completes 746.1 revolutions per mile, based on its 2157.0 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 255/40R19?
The closest size in the chart is 255/45R18, 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 255/40R19 affect my speedometer?
Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 255/45R18 instead of 255/40R19 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.