255/70R15 Tyre Size: Dimensions, Alternatives & Speedometer Effect
255/70R15 is a very wide tall-sidewall tyre size for a 15-inch rim, and the first thing to know about it is the overall diameter: 738.0 mm (29.06 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 178.5 mm tall, the tyre rolls 2318.5 mm around its circumference, and it turns 694.1 times for every mile travelled. If you are here because 255/70R15 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.
In practical terms, 255/70R15 is a very wide tall-sidewall size for a 15-inch wheel. The combination matters because the width sets the contact patch, the aspect ratio sets how much rubber sits between the rim and the road, and the rim diameter sets which wheels the tyre will mount on. Change any one of the three numbers and the overall diameter — and with it the speedometer, the gearing and the ride — moves as well.
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.
255/70R15 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 738.0 mm (29.06 in) |
| Sidewall height | 178.5 mm |
| Circumference | 2318.5 mm (91.28 in) |
| Revolutions per mile | 694.1 |
| Revolutions per kilometre | 431.3 |
| Rim diameter | 15 in (381.0 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 255/70R15 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, 255/70R15 is 16.79% larger in diameter than the widely used 205/55R16 reference size (631.9 mm), so a car moving from that reference size to 255/70R15 would see its speedometer, gearing and ride height shift by that proportion. The ±3% guideline around 255/70R15 runs from 715.9 mm to 760.1 mm: replacements inside that band keep the speedometer error small, and replacements outside it deserve a closer look before fitting.
How to read 255/70R15
The code 255/70R15 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, 70, is the aspect ratio: the sidewall height expressed as a percentage of the width, so 70% of 255 mm gives the 178.5 mm sidewall in the table. The letter R means radial construction, which is the standard for modern road tyres, and the final number, 15, is the rim diameter in inches — the wheel this tyre mounts on. Put together, the diameter is 381.0 mm of rim plus two sidewalls of 178.5 mm, which is 381.0 + 357.0 = 738.0 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 7.0 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 255/70R15
The three sizes in the chart closest to 255/70R15 by overall diameter are listed below. Each one is a real size from the same chart, with its diameter difference worked out against 738.0 mm, so the comparison is exact rather than approximate.
| Alternative size | Diameter (mm) | Difference (mm) | Difference (%) | Actual speed at 60 mph indicated | Rim |
|---|---|---|---|---|---|
| 255/65R16 | 737.9 | -0.1 | -0.01% | 60.0 mph | 16 in |
| 255/60R17 | 737.8 | -0.2 | -0.03% | 60.0 mph | 17 in |
| 255/55R18 | 737.7 | -0.3 | -0.04% | 60.0 mph | 18 in |
255/65R16 measures 737.9 mm across, 0.1 mm shorter than 255/70R15, a difference of 0.01%. Fitted in place of 255/70R15 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 165.8 mm on a 16-inch rim, it turns 694.2 times per mile, and its circumference is 2318.2 mm. That puts it inside the ±3% guideline around 255/70R15, so it is a change the speedometer would barely notice.
255/60R17 measures 737.8 mm across, 0.2 mm shorter than 255/70R15, a difference of 0.03%. Fitted in place of 255/70R15 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 153.0 mm on a 17-inch rim, it turns 694.3 times per mile, and its circumference is 2317.9 mm. That puts it inside the ±3% guideline around 255/70R15, so it is a change the speedometer would barely notice.
255/55R18 measures 737.7 mm across, 0.3 mm shorter than 255/70R15, a difference of 0.04%. Fitted in place of 255/70R15 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 18-inch rim, it turns 694.4 times per mile, and its circumference is 2317.6 mm. That puts it inside the ±3% guideline around 255/70R15, so it is a change the speedometer would barely notice.
For 255/70R15, a useful way to use this table is to start from the diameter difference column, comparing each alternative with 255/65R16 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 16-, 17- and 18-inch rims, and a size on a different rim diameter needs different wheels, not just different tyres.
255/70R15 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, 255/65R16 (737.9 mm against 738.0 mm, a -0.01% change), the numbers work out as follows:
| Indicated speed | Actual speed on 255/65R16 | 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 255/70R15, 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 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 255/70R15
Going wider than 255/70R15 — for example to a 255/65R16 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 178.5 mm sidewall on 255/70R15 is 70.0% 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 tall-sidewall label — 255/70R15 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.
How to check whether 255/70R15 suits your vehicle
The checks for 255/70R15 start from the same place every time: the 738.0 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 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 tyres deserve the same checks, not fewer — the size rules do not relax in cold weather.
The spare deserves the same attention as the four road tyres when 255/70R15 is the size in question. A full-size spare that does not match the 738.0 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 tyres are on, check the pressures again after they have cooled down — a warm fitting reads differently — and use the placard pressure for 255/70R15, checked cold.
Five mistakes to avoid with 255/70R15
With 255/70R15, the errors are rarely about the 255 mm width on its own — they are about the 70% aspect ratio and the 15-inch rim that go with it. People go wrong with 255/70R15 in a handful of repeatable ways. One is to match the width (255 mm) and ignore the aspect ratio (70%) — 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. 255/70R15 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 255/70R15 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 255/70r15 tyre size
If 255/70R15 is your starting point, the closest sizes to compare are 255/65R16 tyre size, 255/60R17 tyre size, 255/55R18 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 255/70R15?
The overall diameter of 255/70R15 is 738.0 mm, which is 29.06 inches. That is the rim diameter of 15 inches (381.0 mm) plus two sidewalls of 178.5 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 255/70R15?
The sidewall height of 255/70R15 is 178.5 mm. It comes from the aspect ratio: 70% of the 255 mm section width is 255 × 70 ÷ 100 = 178.5 mm. That tall-sidewall sidewall is the cushion between the 15-inch rim and the road, and it is what gives 255/70R15 its ride and steering character.
How many revolutions per mile does 255/70R15 do?
255/70R15 turns 694.1 times per mile, or about 431.3 times per kilometre. The figure follows from the circumference of 2318.5 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 255/70R15?
The closest size in the chart to 255/70R15 is 255/65R16, at 737.9 mm — 0.01% smaller in diameter than 255/70R15. At an indicated 60 mph the car would actually be travelling at 60.0 mph on 255/65R16. The next closest are 255/60R17 (-0.03%) and 255/55R18 (-0.04%), and all three are listed with their full figures in the alternatives table above.
Does 255/70R15 affect the speedometer?
Yes — fitting 255/70R15 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 255/70R15 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 715.9 mm and 760.1 mm for 255/70R15 — 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 255/70R15 instead of my current size?
For 255/70R15, 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.