245/55R18 Tire Size: Dimensions, Alternatives & Speedometer Effect
245/55R18 is a wide mid-profile tire size for a 18-inch rim, and the first thing to know about it is the overall diameter: 726.7 mm (28.61 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 134.8 mm tall, the tire rolls 2283.0 mm around its circumference, and it turns 704.9 times for every mile travelled. If you are here because 245/55R18 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 245/55R18 in the wide, mid-profile bracket: the 245 mm tread is paired with a sidewall 55% 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.
245/55R18 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 726.7 mm (28.61 in) |
| Sidewall height | 134.8 mm |
| Circumference | 2283.0 mm (89.88 in) |
| Revolutions per mile | 704.9 |
| Revolutions per kilometre | 438.0 |
| 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 245/55R18 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, 245/55R18 is 15.00% larger in diameter than the widely used 205/55R16 reference size (631.9 mm), so a car moving from that reference size to 245/55R18 would see its speedometer, gearing and ride height shift by that proportion. The ±3% guideline around 245/55R18 runs from 704.9 mm to 748.5 mm: replacements inside that band keep the speedometer error small, and replacements outside it deserve a closer look before fitting.
What 245/55R18 means, number by number
The code 245/55R18 breaks into three parts, and each part is a measurement. The first number, 245, 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, 55, is the aspect ratio: the sidewall height expressed as a percentage of the width, so 55% of 245 mm gives the 134.8 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 134.8 mm, which is 457.2 + 269.6 = 726.7 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 245 mm width adds 12.2 mm to each sidewall and 24.5 mm to the diameter, while widening the section by 10 mm at the same aspect ratio adds 5.5 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.
Alternatives to 245/55R18 from the size chart
The three sizes in the chart closest to 245/55R18 by overall diameter are listed below. Each one is a real size from the same chart, with its diameter difference worked out against 726.7 mm, so the comparison is exact rather than approximate.
| Alternative size | Diameter (mm) | Difference (mm) | Difference (%) | Actual speed at 60 mph indicated | Rim |
|---|---|---|---|---|---|
| 225/60R18 | 727.2 | 0.5 | 0.07% | 60.0 mph | 18 in |
| 275/35R21 | 725.9 | -0.8 | -0.11% | 59.9 mph | 21 in |
| 245/50R19 | 727.6 | 0.9 | 0.12% | 60.1 mph | 19 in |
225/60R18 measures 727.2 mm across, 0.5 mm taller than 245/55R18, a difference of 0.07%. Fitted in place of 245/55R18 it would lift or lower the car by about 0.2 mm — half the diameter change — and at an indicated 60 mph the car would actually be travelling at 60.0 mph. Its sidewall is 135.0 mm on a 18-inch rim, it turns 704.4 times per mile, and its circumference is 2284.6 mm. That puts it inside the ±3% guideline around 245/55R18, so it is a change the speedometer would barely notice.
275/35R21 measures 725.9 mm across, 0.8 mm shorter than 245/55R18, a difference of 0.11%. Fitted in place of 245/55R18 it would lift or lower the car by about 0.4 mm — half the diameter change — and at an indicated 60 mph the car would actually be travelling at 59.9 mph. Its sidewall is 96.2 mm on a 21-inch rim, it turns 705.7 times per mile, and its circumference is 2280.5 mm. That puts it inside the ±3% guideline around 245/55R18, so it is a change the speedometer would barely notice.
245/50R19 measures 727.6 mm across, 0.9 mm taller than 245/55R18, a difference of 0.12%. Fitted in place of 245/55R18 it would lift or lower the car by about 0.4 mm — half the diameter change — and at an indicated 60 mph the car would actually be travelling at 60.1 mph. Its sidewall is 122.5 mm on a 19-inch rim, it turns 704.1 times per mile, and its circumference is 2285.8 mm. That puts it inside the ±3% guideline around 245/55R18, so it is a change the speedometer would barely notice.
For 245/55R18, a useful way to use this table is to start from the diameter difference column, comparing each alternative with 225/60R18 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 18-, 21- and 19-inch rims, and a size on a different rim diameter needs different wheels, not just different tires.
245/55R18 and your speedometer
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, 225/60R18 (727.2 mm against 726.7 mm, a 0.07% change), the numbers work out as follows:
| Indicated speed | Actual speed on 225/60R18 | 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 245/55R18, a 0.07% 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 245/55R18
Swap 245/55R18 for something wider and the contact patch grows: dry grip can improve, but so do rolling resistance and steering weight, and the tire needs clearance that a wider section does not always have. Swap it for something taller and the diameter grows past 726.7 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 726.7 mm, which is exactly why the ±3% guideline is the usual first check.
Ride quality follows the sidewall. The 134.8 mm sidewall on 245/55R18 is 55.0% 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 mid-profile label — 245/55R18 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.
Before fitting 245/55R18: the checks that matter
Before any 245/55R18 is fitted, work through the checks below in order — they are written for this size and its 726.7 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 tires 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 245/55R18: a replacement with a lower load index than the placard asks for is the wrong tire even if the size code matches. Rim width matters too — 245 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.
The spare deserves the same attention as the four road tires when 245/55R18 is the size in question. A full-size spare that does not match the 726.7 mm diameter and the load index of the road set is a problem waiting for a puncture, and a space-saver is deliberately a different size with a speed limit, for reaching a repair only. Once the new tires are on, check the pressures again after they have cooled down — a warm fitting reads differently — and use the placard pressure for 245/55R18, checked cold.
Mistakes people make with 245/55R18
The mistakes below are the ones that catch people out with 245/55R18 in particular, because its 726.7 mm diameter looks close to several neighbouring sizes. People go wrong with 245/55R18 in a handful of repeatable ways. One is to match the width (245 mm) and ignore the aspect ratio (55%) — two tyres with the same width and different aspect ratios have different diameters, so the speedometer and gearing change. Another is to copy the size already on the car without checking the door-jamb placard, which is how a previous owner's wrong choice gets repeated. A third is to buy on size alone and overlook the load index and speed rating, which the placard also specifies. A fourth is to change the size on one axle and not the other, leaving different rolling circumferences front and rear that traction control and all-wheel drive were not designed for. And a fifth is to fit the new tires and keep the old pressures: the correct pressure is the placard figure for the size fitted, checked cold, not a number remembered from the old set.
The last mistake is a quiet one: assuming the size code is the whole specification. 245/55R18 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 245/55R18 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.
Related sizes and tools for 245/55r18 tire size
If 245/55R18 is your starting point, the closest sizes to compare are 225/60R18 tire size, 275/35R21 tire size, 245/50R19 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 245/55R18?
The overall diameter of 245/55R18 is 726.7 mm, which is 28.61 inches. That is the rim diameter of 18 inches (457.2 mm) plus two sidewalls of 134.8 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 245/55R18?
The sidewall height of 245/55R18 is 134.8 mm. It comes from the aspect ratio: 55% of the 245 mm section width is 245 × 55 ÷ 100 = 134.8 mm. That mid-profile sidewall is the cushion between the 18-inch rim and the road, and it is what gives 245/55R18 its ride and steering character.
How many revolutions per mile does 245/55R18 do?
245/55R18 turns 704.9 times per mile, or about 438.0 times per kilometre. The figure follows from the circumference of 2283.0 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 245/55R18?
The closest size in the chart to 245/55R18 is 225/60R18, at 727.2 mm — 0.07% larger in diameter than 245/55R18. At an indicated 60 mph the car would actually be travelling at 60.0 mph on 225/60R18. The next closest are 275/35R21 (-0.11%) and 245/50R19 (0.12%), and all three are listed with their full figures in the alternatives table above.
Does 245/55R18 affect the speedometer?
Yes — fitting 245/55R18 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 245/55R18 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 704.9 mm and 748.5 mm for 245/55R18 — 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 245/55R18 instead of my current size?
For 245/55R18, 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 245 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.