18-inch rim · tyre size comparison
245/45 R18 vs 265/40 R18
265/40 R18 is 8.5 mm (0.33 in) shorter than 245/45 R18, a -1.25% 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
- -8.5 mm
- -0.33 in · -1.25%
- Indicated 70 mph
- 69.1 mph
- Actual speed on 265/40 R18, if the speedometer was accurate on 245/45 R18.
- Sidewall height
- -4.25 mm
- 110.25 mm to 106 mm.
Dimensions side by side
| Measurement | 245/45 R18 | 265/40 R18 | Difference |
|---|---|---|---|
| Overall diameter | 677.7 mm26.68 in | 669.2 mm26.35 in | -8.5 mm (-1.25%)-0.33 in |
| Section width | 245 mm9.65 in | 265 mm10.43 in | +20 mm+0.79 in |
| Sidewall height | 110.25 mm4.34 in | 106 mm4.17 in | -4.25 mm-0.17 in |
| Circumference | 2,129.06 mm83.82 in | 2,102.35 mm82.77 in | -26.7 mm-1.05 in |
| Rim diameter | 457.2 mm18 in | 457.2 mm18 in | 0 mm0 in |
| Revolutions per mile | 755.9 | 765.5 | +9.6 |
| Revolutions per km | 469.69 | 475.66 | +5.97 |
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 265/40 R18
If the speedometer was accurate on 245/45 R18, fitting 265/40 R18 means the car is travelling 1.25% slower than it shows, and the odometer over-records distance by about the same amount.
| Speedometer shows | Actual on 265/40 R18 | Difference |
|---|---|---|
| 30 mph | 29.62 mph | -0.38 mph |
| 50 mph | 49.37 mph | -0.63 mph |
| 60 mph | 59.25 mph | -0.75 mph |
| 70 mph | 69.12 mph | -0.88 mph |
| 50 km/h | 49.37 km/h | -0.63 km/h |
| 100 km/h | 98.75 km/h | -1.25 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 shorter tyre adds to it. Calibration varies by car; these figures are the tyre-size effect only.
What changes from 245/45 R18 to 265/40 R18
Wheels
Both fit 18-inch wheels. Our rim-width starting estimate is 7.5 to 8.5 inches for 245/45 R18 and 8.5 to 9.5 inches for 265/40 R18, so a 8.5-inch wheel sits in both ranges. The exact tyre's approved rim range takes priority.
Sidewall and ride comfort
265/40 R18 has 4.25 mm less sidewall (106 vs 110.25 mm). A shorter sidewall usually sharpens steering response, but rides a little firmer and leaves the rim more exposed to potholes and kerbs.
Contact width
265/40 R18 is 20 mm wider in nominal section width (8.2%). 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 -4.3 mm, and the top of the tyre moves the same distance away from the arch. On the same wheel width and offset, each sidewall moves about 10 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 +1.27%: 3,000 rpm at a given road speed becomes about 3,040 rpm. Gearing gets slightly shorter, so acceleration in each gear sharpens and cruising revs rise.
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 245/45 R18 and 265/40 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
- Plus-sizing wheels: tyre diameter and speedometer changesWork through a 17 to 18 inch wheel change, compare tyre diameters and speedometer effects, and understand why a 3% rule cannot approve fitment.
- Staggered vs square wheels: tyres, rotation and AWDCompare square and staggered wheel setups, calculate front-to-rear tyre diameter, and understand rotation limits and all-wheel-drive requirements.
- How to read a tyre size, load index and sidewall markingsDecode tyre width, aspect ratio, rim diameter, load and speed markings, then calculate sidewall height and diameter using a 225/45 R17 example.
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.
245/45 R18
- BMW 5 Series F10/F11 (2010–2017)Factory and popular setup
- Cupra Formentor 1st gen (KM) (2020–present)Factory trim
- Infiniti G35 Coupe V35 (2003–2007)Factory trim
- Nissan 350Z Z33 (2002–2009)Factory trim
- Nissan 370Z Z34 (2009–2020)Factory trim
- Nissan Z RZ34 (2023–present)Factory trim
- Subaru Legacy BN, 6th gen (2015–2019)Popular setup
265/40 R18
- BMW 3 Series G20 (2019–present)Popular setup
- BMW M3 E92 (2007–2013)Factory trim
- Mercedes-AMG C63 / C63 S W205 / C205 (2015–2021)Factory trim
- Nissan 300ZX / Fairlady Z Z32 (1989–2000)Popular setup
- Porsche 911 997 (2004–2012)Factory trim
- Tesla Model 3 Original / Highland (2017–present)Popular setup
More comparisons
Questions
Can I use 265/40 R18 instead of 245/45 R18?
Both fit 18-inch wheels. The diameter difference is -1.25%, 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 265/40 R18 than 245/45 R18?
Overall diameter is 8.5 mm (0.33 in) smaller: 669.2 vs 677.7 mm. Section width is 20 mm wider, and the sidewall is 4.25 mm shorter. These are nominal figures; real tyres vary by model and rim width.
Will 265/40 R18 change my speedometer reading?
Yes, by 1.25%. If the speedometer was accurate on 245/45 R18, an indicated 60 mph would be about 59.2 mph on 265/40 R18, and an indicated 100 km/h about 98.7 km/h. You would be going slower than the speedometer shows.
Which is better, 245/45 R18 or 265/40 R18?
Neither in general. 245/45 R18 has the taller sidewall, which usually rides more comfortably; 265/40 R18 usually steers more sharply. 265/40 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.