18-inch rim · tyre size comparison

245/40 R18 vs 255/40 R18

255/40 R18 is 8 mm (0.31 in) taller than 245/40 R18, a +1.22% difference in overall diameter. That is inside the ±3% limit commonly used to screen a tire size change; it is a nominal calculation, not a fitment approval.

Diameter difference
+8 mm
+0.31 in · +1.22%
Indicated 70 mph
70.9 mph
Actual speed on 255/40 R18, if the speedometer was accurate on 245/40 R18.
Sidewall height
+4 mm
98 mm to 102 mm.
Compare in the calculator245/40 R18 size guide255/40 R18 size guideSpeedometerWhat changes

Dimensions side by side

245/40 R18 and 255/40 R18 dimensions
Measurement245/40 R18255/40 R18Difference
Overall diameter653.2 mm25.72 in661.2 mm26.03 in+8 mm (+1.22%)+0.31 in
Section width245 mm9.65 in255 mm10.04 in+10 mm+0.39 in
Sidewall height98 mm3.86 in102 mm4.02 in+4 mm+0.16 in
Circumference2,052.09 mm80.79 in2,077.22 mm81.78 in+25.13 mm+0.99 in
Rim diameter457.2 mm18 in457.2 mm18 in0 mm0 in
Revolutions per mile784.25774.76-9.49
Revolutions per km487.31481.41-5.9
245/40 R18255/40 R18
Nominal diameters at the same scale. The inner circle is the rim.

Calculated from the nominal size codes, without load or tread wear. Real tyres vary by model, rim width and pressure, so the exact tyre's published dimensions take priority.

Speedometer with 255/40 R18

If the speedometer was accurate on 245/40 R18, fitting 255/40 R18 means the car is travelling 1.22% faster than it shows, and the odometer under-records distance by about the same amount.

Indicated speed versus calculated road speed on 255/40 R18
Speedometer showsActual on 255/40 R18Difference
30 mph30.37 mph+0.37 mph
50 mph50.61 mph+0.61 mph
60 mph60.73 mph+0.73 mph
70 mph70.86 mph+0.86 mph
50 km/h50.61 km/h+0.61 km/h
100 km/h101.22 km/h+1.22 km/h

UN ECE Regulation 39, used in the UK and EU, does not allow a speedometer to show less than the true speed, so many read slightly high from the factory. A taller tyre uses up some of that margin. Calibration varies by car; these figures are the tyre-size effect only.

What changes from 245/40 R18 to 255/40 R18

  • Wheels

    Both fit 18-inch wheels. Our rim-width starting estimate is 7.5 to 8.5 inches for 245/40 R18 and 8 to 9 inches for 255/40 R18, so a 8 to 8.5-inch wheel sits in both ranges. The exact tyre's approved rim range takes priority.

  • Sidewall and ride comfort

    255/40 R18 has 4 mm more sidewall (102 vs 98 mm). A taller sidewall usually rides a little softer and gives the rim more protection from potholes and kerbs, at the cost of some steering sharpness.

  • Contact width

    255/40 R18 is 10 mm wider in nominal section width (4.1%). A wider tyre usually puts a wider, shorter contact patch on the road, which can help dry grip, but it may follow ruts more, clear standing water less readily and add some rolling resistance.

  • Wheel well clearance

    Ride height changes by about +4 mm, and the top of the tyre moves the same distance closer to the arch. On the same wheel width and offset, each sidewall moves about 5 mm further out towards the arch lip and further in towards the suspension. Check clearance at full steering lock and full suspension compression, especially on a lowered car.

  • Gearing

    The effective final drive changes by -1.21%: 3,000 rpm at a given road speed becomes about 2,960 rpm. Gearing gets slightly longer, so cruising revs drop and acceleration in each gear softens.

  • ABS, stability control and AWD

    These systems compare wheel speeds. Fit the same size on both ends of an axle; with a 1.2% diameter gap, never mix 245/40 R18 and 255/40 R18 on an AWD car, and follow the car maker's rolling-circumference limits, which can be much tighter than 3%.

General tendencies from the nominal numbers, not a test of any tyre. Load index, speed rating, tyre construction and the car's approved sizes matter more than these rules of thumb.

Explained in our guides

Cars in our guides using these sizes

Factory trims and popular community setups from our fitment guides. A popular setup is not a factory approval; each guide explains its trims, axles and clearance limits.

245/40 R18

255/40 R18

More comparisons

245/40 R18 compared

235/40 R18 vs 245/40 R18245/40 R18 vs 265/35 R18245/40 R18 vs 255/35 R18245/35 R18 vs 245/40 R18245/40 R18 vs 245/45 R18

255/40 R18 compared

245/45 R18 vs 255/40 R18255/40 R18 vs 265/35 R18255/35 R18 vs 255/40 R18

Questions

Can I use 255/40 R18 instead of 245/40 R18?

Both fit 18-inch wheels. The diameter difference is +1.22%, inside the ±3% screening limit. That does not approve fitment: check the car maker's approved sizes, load index, speed rating, rim width and clearance first.

How much bigger is 255/40 R18 than 245/40 R18?

Overall diameter is 8 mm (0.31 in) larger: 661.2 vs 653.2 mm. Section width is 10 mm wider, and the sidewall is 4 mm taller. These are nominal figures; real tyres vary by model and rim width.

Will 255/40 R18 change my speedometer reading?

Yes, by 1.22%. If the speedometer was accurate on 245/40 R18, an indicated 60 mph would be about 60.7 mph on 255/40 R18, and an indicated 100 km/h about 101.2 km/h. You would be going faster than the speedometer shows.

Which is better, 245/40 R18 or 255/40 R18?

Neither in general. 255/40 R18 has the taller sidewall, which usually rides more comfortably; 245/40 R18 usually steers more sharply. 255/40 R18 is wider, which can help dry grip; the narrower size is usually better in standing water and snow. The size listed for your car's trim, on its placard or in the handbook, is the safe default.

Check the exact tyre

Size codes give nominal dimensions. Before buying, confirm the size is approved for your car and check the exact tyre's load and speed ratings and published dimensions.

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