245/50R18 Tyre Size: Dimensions, Alternatives & Speedometer Effect
245/50R18 is a wide mid-profile tyre size for a 18-inch rim, and the first thing to know about it is the overall diameter: 702.2 mm (27.65 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 122.5 mm tall, the tyre rolls 2206.0 mm around its circumference, and it turns 729.5 times for every mile travelled. If you are here because 245/50R18 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.
What makes 245/50R18 worth a page of its own is how specific the numbers are. A 245 mm width with a 50% aspect ratio on a 18-inch rim is not interchangeable with the size next to it in a chart: even a 10 mm change in width or a 5-point change in aspect ratio shifts the overall diameter by millimetres that show up on the speedometer. The sections below work through those numbers one at a time.
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.
245/50R18 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 702.2 mm (27.65 in) |
| Sidewall height | 122.5 mm |
| Circumference | 2206.0 mm (86.85 in) |
| Revolutions per mile | 729.5 |
| Revolutions per kilometre | 453.3 |
| 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 245/50R18 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, 245/50R18 is 11.13% larger in diameter than the widely used 205/55R16 reference size (631.9 mm), so a car moving from that reference size to 245/50R18 would see its speedometer, gearing and ride height shift by that proportion. The ±3% guideline around 245/50R18 runs from 681.1 mm to 723.3 mm: replacements inside that band keep the speedometer error small, and replacements outside it deserve a closer look before fitting.
What 245/50R18 means, number by number
The code 245/50R18 breaks into three parts, and each part is a measurement. The first number, 245, 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, 50, is the aspect ratio: the sidewall height expressed as a percentage of the width, so 50% of 245 mm gives the 122.5 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 122.5 mm, which is 457.2 + 245.0 = 702.2 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 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 5.0 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.
Alternatives to 245/50R18 from the size chart
The three sizes in the chart closest to 245/50R18 by overall diameter are listed below. Each one is a real size from the same chart, with its diameter difference worked out against 702.2 mm, so the comparison is exact rather than approximate.
| Alternative size | Diameter (mm) | Difference (mm) | Difference (%) | Actual speed at 60 mph indicated | Rim |
|---|---|---|---|---|---|
| 275/40R19 | 702.6 | 0.4 | 0.06% | 60.0 mph | 19 in |
| 225/60R17 | 701.8 | -0.4 | -0.06% | 60.0 mph | 17 in |
| 245/45R19 | 703.1 | 0.9 | 0.13% | 60.1 mph | 19 in |
275/40R19 measures 702.6 mm across, 0.4 mm taller than 245/50R18, a difference of 0.06%. Fitted in place of 245/50R18 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 110.0 mm on a 19-inch rim, it turns 729.1 times per mile, and its circumference is 2207.3 mm. That puts it inside the ±3% guideline around 245/50R18, so it is a change the speedometer would barely notice.
225/60R17 measures 701.8 mm across, 0.4 mm shorter than 245/50R18, a difference of 0.06%. Fitted in place of 245/50R18 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 17-inch rim, it turns 729.9 times per mile, and its circumference is 2204.8 mm. That puts it inside the ±3% guideline around 245/50R18, so it is a change the speedometer would barely notice.
245/45R19 measures 703.1 mm across, 0.9 mm taller than 245/50R18, a difference of 0.13%. Fitted in place of 245/50R18 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 110.2 mm on a 19-inch rim, it turns 728.6 times per mile, and its circumference is 2208.9 mm. That puts it inside the ±3% guideline around 245/50R18, so it is a change the speedometer would barely notice.
For 245/50R18, a useful way to use this table is to start from the diameter difference column, comparing each alternative with 275/40R19 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 19-, 17- and 19-inch rims, and a size on a different rim diameter needs different wheels, not just different tyres.
245/50R18 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, 275/40R19 (702.6 mm against 702.2 mm, a 0.06% change), the numbers work out as follows:
| Indicated speed | Actual speed on 275/40R19 | 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 245/50R18, a 0.06% 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.
The trade-offs of changing from 245/50R18
Swap 245/50R18 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 702.2 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 702.2 mm, which is exactly why the ±3% guideline is the usual first check.
Ride quality follows the sidewall. The 122.5 mm sidewall on 245/50R18 is 50.0% 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 — 245/50R18 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 245/50R18: the checks that matter
The checks for 245/50R18 start from the same place every time: the 702.2 mm diameter on the placard, and the load and speed ratings that go with it. 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 tyre 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 tyre that is the right size but the wrong load index is still the wrong tyre. Fifth, rim width: a 245 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 tyres deserve the same checks, not fewer — the size rules do not relax in cold weather.
Do not forget the spare when you change to 245/50R18. A full-size spare should match this 702.2 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 245/50R18 is a cold pressure, not a number carried over from the old size.
Five mistakes to avoid with 245/50R18
Most of the trouble with 245/50R18 comes from treating its 702.2 mm diameter as interchangeable with any size that looks similar on paper. People go wrong with 245/50R18 in a handful of repeatable ways. One is to match the width (245 mm) and ignore the aspect ratio (50%) — 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 tyres 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. 245/50R18 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 245/50R18 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.
245/50r18 tyre size: Related sizes and tools
If 245/50R18 is your starting point, the closest sizes to compare are 275/40R19 tyre size, 225/60R17 tyre size, 245/45R19 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 245/50R18?
The overall diameter of 245/50R18 is 702.2 mm, which is 27.65 inches. That is the rim diameter of 18 inches (457.2 mm) plus two sidewalls of 122.5 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 245/50R18?
The sidewall height of 245/50R18 is 122.5 mm. It comes from the aspect ratio: 50% of the 245 mm section width is 245 × 50 ÷ 100 = 122.5 mm. That mid-profile sidewall is the cushion between the 18-inch rim and the road, and it is what gives 245/50R18 its ride and steering character.
How many revolutions per mile does 245/50R18 do?
245/50R18 turns 729.5 times per mile, or about 453.3 times per kilometre. The figure follows from the circumference of 2206.0 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 245/50R18?
The closest size in the chart to 245/50R18 is 275/40R19, at 702.6 mm — 0.06% larger in diameter than 245/50R18. At an indicated 60 mph the car would actually be travelling at 60.0 mph on 275/40R19. The next closest are 225/60R17 (-0.06%) and 245/45R19 (0.13%), and all three are listed with their full figures in the alternatives table above.
Does 245/50R18 affect the speedometer?
Yes — fitting 245/50R18 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 245/50R18 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 681.1 mm and 723.3 mm for 245/50R18 — 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 245/50R18 instead of my current size?
For 245/50R18, 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 tyre professional before fitting a different size.