255/45R18 Tire Size: Dimensions, Alternatives & Speedometer Effect
255/45R18 is a very wide low-profile tire size for a 18-inch rim, and the first thing to know about it is the overall diameter: 686.7 mm (27.04 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 114.8 mm tall, the tire rolls 2157.3 mm around its circumference, and it turns 746.0 times for every mile travelled. If you are here because 255/45R18 is the size on your door-jamb placard, or because it is printed on the sidewall of the tires on your car now, those four numbers are the ones to carry into any comparison.
That puts 255/45R18 in the very wide, low-profile bracket: the 255 mm tread is paired with a sidewall 45% as tall as the tread is wide, on a 18-inch rim. Drivers usually meet this size when they are checking a replacement, comparing a plus-size option, or trying to work out why the speedometer reads slightly differently after a change — and every one of those questions comes back to the same four numbers in the table below.
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.
255/45R18 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 686.7 mm (27.04 in) |
| Sidewall height | 114.8 mm |
| Circumference | 2157.3 mm (84.93 in) |
| Revolutions per mile | 746.0 |
| Revolutions per kilometre | 463.5 |
| Rim diameter | 18 in (457.2 mm) |
These are nominal new-tire 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 tires vary slightly by brand, model, wear and inflation pressure, so two tires marked 255/45R18 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, 255/45R18 is 8.67% larger in diameter than the widely used 205/55R16 reference size (631.9 mm), so a car moving from that reference size to 255/45R18 would see its speedometer, gearing and ride height shift by that proportion. The ±3% guideline around 255/45R18 runs from 666.1 mm to 707.3 mm: replacements inside that band keep the speedometer error small, and replacements outside it deserve a closer look before fitting.
What 255/45R18 means, number by number
The code 255/45R18 breaks into three parts, and each part is a measurement. The first number, 255, is the section width in millimetres — the width of the tire from one sidewall to the other at its widest point, measured on its approved rim width. The second number, 45, is the aspect ratio: the sidewall height expressed as a percentage of the width, so 45% of 255 mm gives the 114.8 mm sidewall in the table. The letter R means radial construction, which is the standard for modern road tires, and the final number, 18, is the rim diameter in inches — the wheel this tire mounts on. Put together, the diameter is 457.2 mm of rim plus two sidewalls of 114.8 mm, which is 457.2 + 229.6 = 686.7 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 4.5 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.
Three sizes closest to 255/45R18
The three sizes in the chart closest to 255/45R18 by overall diameter are listed below. Each one is a real size from the same chart, with its diameter difference worked out against 686.7 mm, so the comparison is exact rather than approximate.
| Alternative size | Diameter (mm) | Difference (mm) | Difference (%) | Actual speed at 60 mph indicated | Rim |
|---|---|---|---|---|---|
| 255/50R17 | 686.8 | 0.1 | 0.01% | 60.0 mph | 17 in |
| 255/40R19 | 686.6 | -0.1 | -0.01% | 60.0 mph | 19 in |
| 255/55R16 | 686.9 | 0.2 | 0.03% | 60.0 mph | 16 in |
255/50R17 measures 686.8 mm across, 0.1 mm taller than 255/45R18, a difference of 0.01%. Fitted in place of 255/45R18 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 127.5 mm on a 17-inch rim, it turns 745.9 times per mile, and its circumference is 2157.6 mm. That puts it inside the ±3% guideline around 255/45R18, so it is a change the speedometer would barely notice.
255/40R19 measures 686.6 mm across, 0.1 mm shorter than 255/45R18, a difference of 0.01%. Fitted in place of 255/45R18 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 102.0 mm on a 19-inch rim, it turns 746.1 times per mile, and its circumference is 2157.0 mm. That puts it inside the ±3% guideline around 255/45R18, so it is a change the speedometer would barely notice.
255/55R16 measures 686.9 mm across, 0.2 mm taller than 255/45R18, a difference of 0.03%. Fitted in place of 255/45R18 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 140.2 mm on a 16-inch rim, it turns 745.8 times per mile, and its circumference is 2158.0 mm. That puts it inside the ±3% guideline around 255/45R18, so it is a change the speedometer would barely notice.
For 255/45R18, a useful way to use this table is to start from the diameter difference column, comparing each alternative with 255/50R17 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 16-inch rims, and a size on a different rim diameter needs different wheels, not just different tires.
What 255/45R18 does to speedometer readings
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, 255/50R17 (686.8 mm against 686.7 mm, a 0.01% change), the numbers work out as follows:
| Indicated speed | Actual speed on 255/50R17 | 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 255/45R18, a 0.01% 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 tire 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 255/45R18
Going wider than 255/45R18 — for example to a 255/50R17 if its width is greater — puts more rubber on the road, which can sharpen dry grip and steering response, but it also adds rolling resistance and can make the steering feel heavier, and the extra width needs clearance at the arch, the suspension and full steering lock. Going taller raises the overall diameter, lifts the car by half the diameter increase, lengthens the gearing slightly and slows the speedometer reading relative to actual speed. Going narrower or shorter does the opposite: lighter steering and a little less rolling resistance, but a smaller contact patch and a speedometer that reads high. None of these effects is large for a change inside the ±3% guideline, which is why that guideline exists — it keeps the gearing, the speedometer and the clearance close to what the vehicle was designed around.
Ride quality follows the sidewall. The 114.8 mm sidewall on 255/45R18 is 45.0% 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 — 255/45R18 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 tire will perform as its size suggests it should.
How to check whether 255/45R18 suits your vehicle
Before any 255/45R18 is fitted, work through the checks below in order — they are written for this size and its 686.7 mm diameter. Do the checks in order. First, the door-jamb placard: it is the manufacturer's statement of which size, pressures, load index and speed rating the vehicle was built around. Second, the owner's manual, which confirms the placard and covers the spare. Third, the sidewall of the tire on the car now — if it reads something other than the placard size, treat that as a question to answer, not as the specification. Fourth, the load index and speed rating on the replacement: both must meet or exceed what the placard requires, because a tire that is the right size but the wrong load index is still the wrong tire. Fifth, rim width: a 255 mm section mounts correctly only on a rim within its approved width range. Sixth, the pressure monitoring system, which may need a reset after any change in rolling circumference. Winter tires deserve the same checks, not fewer — the size rules do not relax in cold weather.
Do not forget the spare when you change to 255/45R18. A full-size spare should match this 686.7 mm size and the load index of the road tires, 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 tires have cooled: a tire fitted warm can read differently cold, and the placard pressure for 255/45R18 is a cold pressure, not a number carried over from the old size.
Common mistakes with 255/45R18
With 255/45R18, the errors are rarely about the 255 mm width on its own — they are about the 45% aspect ratio and the 18-inch rim that go with it. Five mistakes account for most of the trouble with a size like 255/45R18. Matching only the 255 mm width while the 45% aspect ratio differs, which changes the diameter. Copying the size moulded on the current tires 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 tires 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. 255/45R18 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 tires marked 255/45R18 can still be very different tires. 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 tire.
255/45r18 tire size: Related sizes and tools
If 255/45R18 is your starting point, the closest sizes to compare are 255/50R17 tire size, 255/40R19 tire size, 255/55R16 tire size. The tire 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 255/45R18?
The overall diameter of 255/45R18 is 686.7 mm, which is 27.04 inches. That is the rim diameter of 18 inches (457.2 mm) plus two sidewalls of 114.8 mm each. It is a nominal new-tire figure; real tires vary slightly by brand, model, wear and inflation pressure.
What is the sidewall height of 255/45R18?
The sidewall height of 255/45R18 is 114.8 mm. It comes from the aspect ratio: 45% of the 255 mm section width is 255 × 45 ÷ 100 = 114.8 mm. That low-profile sidewall is the cushion between the 18-inch rim and the road, and it is what gives 255/45R18 its ride and steering character.
How many revolutions per mile does 255/45R18 do?
255/45R18 turns 746.0 times per mile, or about 463.5 times per kilometre. The figure follows from the circumference of 2157.3 mm: one mile is 63,360 inches, and each revolution covers one circumference. A replacement tire 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 255/45R18?
The closest size in the chart to 255/45R18 is 255/50R17, at 686.8 mm — 0.01% larger in diameter than 255/45R18. At an indicated 60 mph the car would actually be travelling at 60.0 mph on 255/50R17. The next closest are 255/40R19 (-0.01%) and 255/55R16 (0.03%), and all three are listed with their full figures in the alternatives table above.
Does 255/45R18 affect the speedometer?
Yes — fitting 255/45R18 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 255/45R18 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 666.1 mm and 707.3 mm for 255/45R18 — 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 255/45R18 instead of my current size?
For 255/45R18, 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 tire professional before fitting a different size.