255/40R18 Tyre Size: Dimensions, Alternatives & Speedometer Effect
255/40R18 is a very wide low-profile tyre size for a 18-inch rim, and the first thing to know about it is the overall diameter: 661.2 mm (26.03 inches). That single figure drives almost everything else on this page — the speedometer reading, the gearing, the ground clearance and which sizes can replace it without changing how the car behaves. The sidewall stands 102.0 mm tall, the tyre rolls 2077.2 mm around its circumference, and it turns 774.8 times for every mile travelled. If you are here because 255/40R18 is the size on your door-jamb placard, or because it is printed on the sidewall of the tyres on your car now, those four numbers are the ones to carry into any comparison.
For a 18-inch wheel, 255/40R18 sits in the very wide low-profile group. That combination of width, sidewall height and rim size decides the overall diameter of 661.2 mm, which is the single number that governs speedometer accuracy, gearing and ground clearance. The rest of this page decodes the code, lists the measured dimensions, and compares the three closest sizes in the chart so you can judge any change before you buy.
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.
255/40R18 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 661.2 mm (26.03 in) |
| Sidewall height | 102.0 mm |
| Circumference | 2077.2 mm (81.78 in) |
| Revolutions per mile | 774.8 |
| Revolutions per kilometre | 481.4 |
| Rim diameter | 18 in (457.2 mm) |
These are nominal new-tyre figures, worked out from the size code itself: the diameter is the rim diameter plus twice the sidewall height, the circumference is pi times the diameter, and the revolutions per mile follow from the circumference. Real tyres vary slightly by brand, model, wear and inflation pressure, so two tyres marked 255/40R18 can differ by a millimetre or two in practice — the table gives the standard computed values, which are the right ones to use for comparisons. For scale, 255/40R18 is 4.64% larger in diameter than the widely used 205/55R16 reference size (631.9 mm), so a car moving from that reference size to 255/40R18 would see its speedometer, gearing and ride height shift by that proportion. The ±3% guideline around 255/40R18 runs from 641.4 mm to 681.0 mm: replacements inside that band keep the speedometer error small, and replacements outside it deserve a closer look before fitting.
How to read 255/40R18
The code 255/40R18 breaks into three parts, and each part is a measurement. The first number, 255, is the section width in millimetres — the width of the tyre from one sidewall to the other at its widest point, measured on its approved rim width. The second number, 40, is the aspect ratio: the sidewall height expressed as a percentage of the width, so 40% of 255 mm gives the 102.0 mm sidewall in the table. The letter R means radial construction, which is the standard for modern road tyres, and the final number, 18, is the rim diameter in inches — the wheel this tyre mounts on. Put together, the diameter is 457.2 mm of rim plus two sidewalls of 102.0 mm, which is 457.2 + 204.0 = 661.2 mm, matching the table. That arithmetic is worth doing once by hand, because it shows why two sizes that look similar can measure differently: raising the aspect ratio by 5 points on a 255 mm width adds 12.8 mm to each sidewall and 25.5 mm to the diameter, while widening the section by 10 mm at the same aspect ratio adds 4.0 mm per sidewall. The size code tells you nothing about load index or speed rating — those are separate markings on the sidewall, and the placard specifies the minimum for both.
Closest alternatives to 255/40R18
The three sizes in the chart closest to 255/40R18 by overall diameter are listed below. Each one is a real size from the same chart, with its diameter difference worked out against 661.2 mm, so the comparison is exact rather than approximate.
| Alternative size | Diameter (mm) | Difference (mm) | Difference (%) | Actual speed at 60 mph indicated | Rim |
|---|---|---|---|---|---|
| 255/45R17 | 661.3 | 0.1 | 0.02% | 60.0 mph | 17 in |
| 255/35R19 | 661.1 | -0.1 | -0.02% | 60.0 mph | 19 in |
| 175/80R15 | 661.0 | -0.2 | -0.03% | 60.0 mph | 15 in |
255/45R17 measures 661.3 mm across, 0.1 mm taller than 255/40R18, a difference of 0.02%. Fitted in place of 255/40R18 it would lift or lower the car by about 0.0 mm — half the diameter change — and at an indicated 60 mph the car would actually be travelling at 60.0 mph. Its sidewall is 114.8 mm on a 17-inch rim, it turns 774.6 times per mile, and its circumference is 2077.5 mm. That puts it inside the ±3% guideline around 255/40R18, so it is a change the speedometer would barely notice.
255/35R19 measures 661.1 mm across, 0.1 mm shorter than 255/40R18, a difference of 0.02%. Fitted in place of 255/40R18 it would lift or lower the car by about 0.1 mm — half the diameter change — and at an indicated 60 mph the car would actually be travelling at 60.0 mph. Its sidewall is 89.2 mm on a 19-inch rim, it turns 774.9 times per mile, and its circumference is 2076.9 mm. That puts it inside the ±3% guideline around 255/40R18, so it is a change the speedometer would barely notice.
175/80R15 measures 661.0 mm across, 0.2 mm shorter than 255/40R18, a difference of 0.03%. Fitted in place of 255/40R18 it would lift or lower the car by about 0.1 mm — half the diameter change — and at an indicated 60 mph the car would actually be travelling at 60.0 mph. Its sidewall is 140.0 mm on a 15-inch rim, it turns 775.0 times per mile, and its circumference is 2076.6 mm. That puts it inside the ±3% guideline around 255/40R18, so it is a change the speedometer would barely notice.
For 255/40R18, a useful way to use this table is to start from the diameter difference column, comparing each alternative with 255/45R17 first. Anything inside ±3% keeps the speedometer, the gearing and the ride height close to the original, and most drivers would not notice the change in daily driving. Anything outside that band changes the car's behaviour in ways that are easy to measure — the speedometer error grows, the gearing lengthens or shortens, and the ride height moves — so it needs the placard check and a professional's confirmation before fitting. Rim size matters here too: the alternatives above use 17-, 19- and 15-inch rims, and a size on a different rim diameter needs different wheels, not just different tyres.
Speedometer effect of 255/40R18
The speedometer is calibrated for the original diameter, so any change in diameter changes what the dial shows. The formula is straightforward: error % = (new diameter − old diameter) ÷ old diameter × 100, and actual speed = indicated speed × (new diameter ÷ old diameter). For the closest alternative, 255/45R17 (661.3 mm against 661.2 mm, a 0.02% change), the numbers work out as follows:
| Indicated speed | Actual speed on 255/45R17 | Actual at +3% diameter | Actual at −3% diameter |
|---|---|---|---|
| 30 mph | 30.0 mph | 30.9 mph | 29.1 mph |
| 50 mph | 50.0 mph | 51.5 mph | 48.5 mph |
| 70 mph | 70.0 mph | 72.1 mph | 67.9 mph |
For 255/40R18, a 0.02% diameter change sounds small, and at 30 mph it is — the difference is under a mile per hour either way. At 70 mph the same percentage is worth a little more, which is why motorway driving is where a size change shows up first. The +3% and −3% columns show the edges of the usual guideline: at the +3% edge the car is travelling about 2 mph faster than the dial at 70 mph, and at the −3% edge about 2 mph slower. That is the reason the guideline is ±3% rather than a wider band — beyond it, the error at motorway speed starts to matter for speed limits and for the odometer, which records distance from the same wheel-speed signal and so drifts by the same percentage. A tyre change also affects the odometer reading, the gearing and, on some cars, the calibration of driver-assistance systems that use wheel speed.
Going wider or taller than 255/40R18
Swap 255/40R18 for something wider and the contact patch grows: dry grip can improve, but so do rolling resistance and steering weight, and the tyre needs clearance that a wider section does not always have. Swap it for something taller and the diameter grows past 661.2 mm, ride height rises by half the gain, gearing lengthens and the speedometer under-reads. Swap it for something narrower or shorter and the opposite happens — the speedometer over-reads, gearing shortens, and the smaller sidewall can make the ride feel busier. These are qualitative effects, not promises: the size of each effect depends on how far the new diameter sits from 661.2 mm, which is exactly why the ±3% guideline is the usual first check.
Ride quality follows the sidewall. The 102.0 mm sidewall on 255/40R18 is 40.0% of the section width, and it is the cushion between the rim and the road: a taller sidewall absorbs small bumps and protects the rim from pothole damage, while a shorter sidewall gives sharper steering response and less flex in corners. That is the trade behind the low-profile label — 255/40R18 is neither the softest nor the sharpest option in its rim size, and moving to a much lower aspect ratio on the same rim would firm the ride noticeably, while moving to a much taller one would soften it. Fuel use moves with rolling resistance, which tends to rise with width and fall slightly with a taller, more flexible sidewall, but the differences between sizes inside the ±3% band are small compared with the differences that pressure, tread pattern and driving style make. Keep the pressures at the placard figure for the size fitted, checked cold, and the tyre will perform as its size suggests it should.
Before fitting 255/40R18: the checks that matter
For 255/40R18, the checks below are the ones that decide whether a replacement is sensible, starting from its 661.2 mm diameter. The placard on the driver's door jamb is the first document to find. It gives the manufacturer-specified size, the recommended cold pressures front and rear, and usually the load index and speed rating the vehicle needs. The owner's manual repeats that information and adds the spare-tyre size, which is often different. Compare both with the size moulded on the sidewall of the tyres fitted now: if they do not match, find out why before ordering replacements, because the fitted size may have been a previous owner's choice rather than the manufacturer's. Load index and speed rating matter as much as the three numbers in 255/40R18: a replacement with a lower load index than the placard asks for is the wrong tyre even if the size code matches. Rim width matters too — 255 mm of section width needs a rim in the approved width range for that size. And if the car has tyre pressure monitoring, remember that a circumference change can affect it, so budget for a reset or relearn.
Do not forget the spare when you change to 255/40R18. A full-size spare should match this 661.2 mm size and the load index of the road tyres, while a space-saver is a different size with its own speed limit, meant only to get the car to a repair. After fitting, re-check the pressures once the tyres have cooled: a tyre fitted warm can read differently cold, and the placard pressure for 255/40R18 is a cold pressure, not a number carried over from the old size.
Five mistakes to avoid with 255/40R18
Most of the trouble with 255/40R18 comes from treating its 661.2 mm diameter as interchangeable with any size that looks similar on paper. Five mistakes account for most of the trouble with a size like 255/40R18. Matching only the 255 mm width while the 40% aspect ratio differs, which changes the diameter. Copying the size moulded on the current tyres instead of the door-jamb placard, which repeats whatever the last owner fitted. Buying the right size with the wrong load index or speed rating. Changing size on one axle only, so the front and rear roll at different circumferences and the stability and traction systems see a mismatch. And fitting the new tyres at the old pressures instead of the placard pressure for the new size, checked when cold. Each of these is easy to avoid by working from the placard and the four numbers in the table above, in that order.
The last mistake is a quiet one: assuming the size code is the whole specification. 255/40R18 fixes the width, the aspect ratio and the rim, but it does not fix the load index, the speed rating, the tread pattern or the compound, and two tyres marked 255/40R18 can still be very different tyres. Read the full sidewall, match the load index and speed rating to the placard, and choose the tread and compound for the driving the car actually does. The size gets you to the right shelf; the rest of the sidewall gets you to the right tyre.
Related sizes and tools for 255/40r18 tyre size
If 255/40R18 is your starting point, the closest sizes to compare are 255/45R17 tyre size, 255/35R19 tyre size, 175/80R15 tyre size. The tyre size calculator page explains how to decode any size code and work out its dimensions, and the tyre size calculator tool does the arithmetic for any size in the chart. The tyre sizes hub lists every size covered on this site, and the tyre size chart shows the full computed table side by side.
Frequently asked questions
What is the overall diameter of 255/40R18?
The overall diameter of 255/40R18 is 661.2 mm, which is 26.03 inches. That is the rim diameter of 18 inches (457.2 mm) plus two sidewalls of 102.0 mm each. It is a nominal new-tyre figure; real tyres vary slightly by brand, model, wear and inflation pressure.
What is the sidewall height of 255/40R18?
The sidewall height of 255/40R18 is 102.0 mm. It comes from the aspect ratio: 40% of the 255 mm section width is 255 × 40 ÷ 100 = 102.0 mm. That low-profile sidewall is the cushion between the 18-inch rim and the road, and it is what gives 255/40R18 its ride and steering character.
How many revolutions per mile does 255/40R18 do?
255/40R18 turns 774.8 times per mile, or about 481.4 times per kilometre. The figure follows from the circumference of 2077.2 mm: one mile is 63,360 inches, and each revolution covers one circumference. A replacement tyre with a different circumference turns a different number of times per mile, which is why the speedometer and odometer readings move when the diameter changes.
What is the closest alternative size to 255/40R18?
The closest size in the chart to 255/40R18 is 255/45R17, at 661.3 mm — 0.02% larger in diameter than 255/40R18. At an indicated 60 mph the car would actually be travelling at 60.0 mph on 255/45R17. The next closest are 255/35R19 (-0.02%) and 175/80R15 (-0.03%), and all three are listed with their full figures in the alternatives table above.
Does 255/40R18 affect the speedometer?
Yes — fitting 255/40R18 in place of a different original size changes the reading, because the speedometer is calibrated for the diameter the car was built with, so fitting 255/40R18 in place of a size with a different diameter changes the reading by the diameter difference as a percentage. A replacement within ±3% of the original diameter — between 641.4 mm and 681.0 mm for 255/40R18 — keeps the error small; outside that band the error at motorway speed becomes noticeable, and the odometer drifts by the same percentage.
Can I fit 255/40R18 instead of my current size?
For 255/40R18, only if the diameter stays within about ±3% of the size on your door-jamb placard, the load index and speed rating meet what the placard specifies, the rim width suits a 255 mm section, and there is clearance at the arch, the suspension and full steering lock. This page gives the measurements so you can make that comparison; it does not approve a fitment. Check the placard and confirm with a tyre professional before fitting a different size.