255/40R17 Tire Size: Dimensions, Alternatives & Speedometer Effect
255/40R17 is a very wide low-profile tire size for a 17-inch rim, and the first thing to know about it is the overall diameter: 635.8 mm (25.03 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 tire rolls 1997.4 mm around its circumference, and it turns 805.7 times for every mile travelled. If you are here because 255/40R17 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.
What makes 255/40R17 worth a page of its own is how specific the numbers are. A 255 mm width with a 40% aspect ratio on a 17-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.
255/40R17 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 635.8 mm (25.03 in) |
| Sidewall height | 102.0 mm |
| Circumference | 1997.4 mm (78.64 in) |
| Revolutions per mile | 805.7 |
| Revolutions per kilometre | 500.6 |
| Rim diameter | 17 in (431.8 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 255/40R17 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, 255/40R17 is 0.62% larger in diameter than the widely used 205/55R16 reference size (631.9 mm), so a car moving from that reference size to 255/40R17 would see its speedometer, gearing and ride height shift by that proportion. The ±3% guideline around 255/40R17 runs from 616.7 mm to 654.9 mm: replacements inside that band keep the speedometer error small, and replacements outside it deserve a closer look before fitting.
What 255/40R17 means, number by number
The code 255/40R17 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, 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 tires, and the final number, 17, is the rim diameter in inches — the wheel this tire mounts on. Put together, the diameter is 431.8 mm of rim plus two sidewalls of 102.0 mm, which is 431.8 + 204.0 = 635.8 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.
Alternatives to 255/40R17 from the size chart
The three sizes in the chart closest to 255/40R17 by overall diameter are listed below. Each one is a real size from the same chart, with its diameter difference worked out against 635.8 mm, so the comparison is exact rather than approximate.
| Alternative size | Diameter (mm) | Difference (mm) | Difference (%) | Actual speed at 60 mph indicated | Rim |
|---|---|---|---|---|---|
| 255/35R18 | 635.7 | -0.1 | -0.02% | 60.0 mph | 18 in |
| 205/50R17 | 636.8 | 1.0 | 0.16% | 60.1 mph | 17 in |
| 195/65R15 | 634.5 | -1.3 | -0.20% | 59.9 mph | 15 in |
255/35R18 measures 635.7 mm across, 0.1 mm shorter than 255/40R17, a difference of 0.02%. Fitted in place of 255/40R17 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 89.2 mm on a 18-inch rim, it turns 805.8 times per mile, and its circumference is 1997.1 mm. That puts it inside the ±3% guideline around 255/40R17, so it is a change the speedometer would barely notice.
205/50R17 measures 636.8 mm across, 1.0 mm taller than 255/40R17, a difference of 0.16%. Fitted in place of 255/40R17 it would lift or lower the car by about 0.5 mm — half the diameter change — and at an indicated 60 mph the car would actually be travelling at 60.1 mph. Its sidewall is 102.5 mm on a 17-inch rim, it turns 804.4 times per mile, and its circumference is 2000.6 mm. That puts it inside the ±3% guideline around 255/40R17, so it is a change the speedometer would barely notice.
195/65R15 measures 634.5 mm across, 1.3 mm shorter than 255/40R17, a difference of 0.20%. Fitted in place of 255/40R17 it would lift or lower the car by about 0.6 mm — half the diameter change — and at an indicated 60 mph the car would actually be travelling at 59.9 mph. Its sidewall is 126.8 mm on a 15-inch rim, it turns 807.4 times per mile, and its circumference is 1993.3 mm. That puts it inside the ±3% guideline around 255/40R17, so it is a change the speedometer would barely notice.
For 255/40R17, a useful way to use this table is to start from the diameter difference column, comparing each alternative with 255/35R18 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-, 17- and 15-inch rims, and a size on a different rim diameter needs different wheels, not just different tires.
255/40R17 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, 255/35R18 (635.7 mm against 635.8 mm, a -0.02% change), the numbers work out as follows:
| Indicated speed | Actual speed on 255/35R18 | 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 255/40R17, 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 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 255/40R17
Width, height and diameter each pull in a different direction. More width than the 255 mm of 255/40R17 generally means more grip and more rolling resistance, plus a wider tire that needs more room at the wheel arch and at steering lock. More overall diameter than 635.8 mm lifts the vehicle, lengthens the gearing and slows the speedometer; less diameter drops the car, shortens the gearing and makes the speedometer read fast. A taller sidewall than the 102.0 mm here absorbs bumps better and protects the rim, while a shorter sidewall sharpens steering response at the cost of a firmer ride. Staying within ±3% of the original diameter keeps all of these changes small enough that the vehicle behaves much as it did before.
Ride quality follows the sidewall. The 102.0 mm sidewall on 255/40R17 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 — 255/40R17 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 255/40R17 suits your vehicle
Before any 255/40R17 is fitted, work through the checks below in order — they are written for this size and its 635.8 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 255/40R17: 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 — 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 255/40R17. A full-size spare should match this 635.8 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 255/40R17 is a cold pressure, not a number carried over from the old size.
Five mistakes to avoid with 255/40R17
The mistakes below are the ones that catch people out with 255/40R17 in particular, because its 635.8 mm diameter looks close to several neighbouring sizes. The most common mistake is reading only the width. A driver sees 255 mm and assumes any 255-width tire will do, but 255/40R17 also fixes the aspect ratio at 40% and the rim at 17 inches, and a different aspect ratio on the same width changes the diameter by several millimetres. The second mistake is trusting the size on the car now instead of the placard — a previous owner may have fitted the wrong size, and copying it repeats the error. The third is ignoring the load index and speed rating, which are separate from the size code and just as important. The fourth is fitting a different size on one axle only, which leaves the car with two different rolling circumferences and can confuse traction control, ABS and all-wheel-drive systems. The fifth is forgetting pressure: a new size needs the placard pressure for that size, not the pressure the old tires happened to be running at.
The last mistake is a quiet one: assuming the size code is the whole specification. 255/40R17 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 255/40R17 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.
255/40r17 tire size: Related sizes and tools
If 255/40R17 is your starting point, the closest sizes to compare are 255/35R18 tire size, 205/50R17 tire size, 195/65R15 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 255/40R17?
The overall diameter of 255/40R17 is 635.8 mm, which is 25.03 inches. That is the rim diameter of 17 inches (431.8 mm) plus two sidewalls of 102.0 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 255/40R17?
The sidewall height of 255/40R17 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 17-inch rim and the road, and it is what gives 255/40R17 its ride and steering character.
How many revolutions per mile does 255/40R17 do?
255/40R17 turns 805.7 times per mile, or about 500.6 times per kilometre. The figure follows from the circumference of 1997.4 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 255/40R17?
The closest size in the chart to 255/40R17 is 255/35R18, at 635.7 mm — 0.02% smaller in diameter than 255/40R17. At an indicated 60 mph the car would actually be travelling at 60.0 mph on 255/35R18. The next closest are 205/50R17 (0.16%) and 195/65R15 (-0.20%), and all three are listed with their full figures in the alternatives table above.
Does 255/40R17 affect the speedometer?
Yes — fitting 255/40R17 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 255/40R17 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 616.7 mm and 654.9 mm for 255/40R17 — 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 255/40R17 instead of my current size?
For 255/40R17, 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.