18-inch rim · tyre size comparison

245/40 R18 vs 245/45 R18

245/45 R18 is 24.5 mm (0.96 in) taller than 245/40 R18, a +3.75% difference in overall diameter. That is just outside the ±3% limit commonly used to screen a tire size change, so expect a noticeable speedometer error; it is a nominal calculation, not a fitment approval.

Diameter difference
+24.5 mm
+0.96 in · +3.75%
Indicated 70 mph
72.6 mph
Actual speed on 245/45 R18, if the speedometer was accurate on 245/40 R18.
Sidewall height
+12.25 mm
98 mm to 110.25 mm.
Compare in the calculator245/40 R18 size guide245/45 R18 size guideSpeedometerWhat changes

Dimensions side by side

245/40 R18 and 245/45 R18 dimensions
Measurement245/40 R18245/45 R18Difference
Overall diameter653.2 mm25.72 in677.7 mm26.68 in+24.5 mm (+3.75%)+0.96 in
Section width245 mm9.65 in245 mm9.65 in0 mm0 in
Sidewall height98 mm3.86 in110.25 mm4.34 in+12.25 mm+0.48 in
Circumference2,052.09 mm80.79 in2,129.06 mm83.82 in+76.97 mm+3.03 in
Rim diameter457.2 mm18 in457.2 mm18 in0 mm0 in
Revolutions per mile784.25755.9-28.35
Revolutions per km487.31469.69-17.62
245/40 R18245/45 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 245/45 R18

If the speedometer was accurate on 245/40 R18, fitting 245/45 R18 means the car is travelling 3.75% faster than it shows, and the odometer under-records distance by about the same amount.

Indicated speed versus calculated road speed on 245/45 R18
Speedometer showsActual on 245/45 R18Difference
30 mph31.13 mph+1.13 mph
50 mph51.88 mph+1.88 mph
60 mph62.25 mph+2.25 mph
70 mph72.63 mph+2.63 mph
50 km/h51.88 km/h+1.88 km/h
100 km/h103.75 km/h+3.75 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 245/45 R18

  • Wheels

    Both fit 18-inch wheels. Our rim-width starting estimate is 7.5 to 8.5 inches for 245/40 R18 and 7.5 to 8.5 inches for 245/45 R18, so a 7.5 to 8.5-inch wheel sits in both ranges. The exact tyre's approved rim range takes priority.

  • Sidewall and ride comfort

    245/45 R18 has 12.25 mm more sidewall (110.25 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

    Both are 245 mm in nominal section width, so the tread is about as wide. Contact patch area is set mostly by load and pressure, so grip differences come more from the tyre model than this size change.

  • Wheel well clearance

    Ride height changes by about +12.3 mm, and the top of the tyre moves the same distance closer to the arch. On the same wheel, the sidewalls stay in roughly the same place side to side. Check clearance at full steering lock and full suspension compression, especially on a lowered car.

  • Gearing

    The effective final drive changes by -3.62%: 3,000 rpm at a given road speed becomes about 2,890 rpm. Gearing gets noticeably 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 3.8% diameter gap, never mix 245/40 R18 and 245/45 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

245/45 R18

More comparisons

245/40 R18 compared

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

245/45 R18 compared

225/50 R18 vs 245/45 R18245/45 R18 vs 265/40 R18235/45 R18 vs 245/45 R18245/45 R18 vs 255/45 R18235/50 R18 vs 245/45 R18245/45 R18 vs 255/40 R18245/45 R18 vs 245/50 R18

Questions

Can I use 245/45 R18 instead of 245/40 R18?

Both fit 18-inch wheels. The diameter difference is +3.75%, just outside the ±3% screening limit, so most fitters would treat it as a different size class rather than a direct replacement. 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 245/45 R18 than 245/40 R18?

Overall diameter is 24.5 mm (0.96 in) larger: 677.7 vs 653.2 mm. Section width is the same, and the sidewall is 12.25 mm taller. These are nominal figures; real tyres vary by model and rim width.

Will 245/45 R18 change my speedometer reading?

Yes, by 3.75%. If the speedometer was accurate on 245/40 R18, an indicated 60 mph would be about 62.3 mph on 245/45 R18, and an indicated 100 km/h about 103.8 km/h. You would be going faster than the speedometer shows.

Which is better, 245/40 R18 or 245/45 R18?

Neither in general. 245/45 R18 has the taller sidewall, which usually rides more comfortably; 245/40 R18 usually steers more sharply. 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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