225/40 R18 vs 235/40 R18

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.

If the speedometer was accurate on 225/40 R18, fitting 235/40 R18 means the car is travelling 1.26% faster than it shows, and the odometer under-records distance by about the same amount.

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.

• Wheels

  Both fit 18-inch wheels. Our rim-width starting estimate is 7 to 8 inches for 225/40 R18 and 7.5 to 8.5 inches for 235/40 R18, so a 7.5 to 8-inch wheel sits in both ranges. The exact tyre's approved rim range takes priority.

• Sidewall and ride comfort

  235/40 R18 has 4 mm more sidewall (94 vs 90 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

  235/40 R18 is 10 mm wider in nominal section width (4.4%). 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 -1.24%: 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 1.3% diameter gap, never mix 225/40 R18 and 235/40 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.

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.

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.