Tire size
255/40 R18
Full dimensions and a live comparison calculator for the 255/40 R18 tire size.
Overall diameter
661.2 mm
26.03″
Sidewall height
102.0 mm
Circumference
2.077 m
Revs / km
481.4
Section width
255 mm
Aspect ratio
40%
Rim diameter
18″
457.2 mm
Radius
330.6 mm
What does 255/40 R18 mean?
The 255 is the section width in millimetres, measured sidewall to sidewall. The 40 is the aspect ratio — sidewall height as a percentage of width, so the sidewall here is 102.0 mm. The R18 means a radial-construction tire designed for a 18-inch wheel rim. Together these give an overall diameter of 661 mm.
Compare 255/40 R18 with another size
Current Tire
New Tire
Ground line · scaled comparison
Slight Difference
Within ±5% — usable, recalibration recommended
Diameter change
-26.9 mm
-4.07%
Speedometer at 100
95.9 km/h
-4.07% error
Ground clearance
-13.5 mm
ride height delta
Sidewall change
-0.8 mm
revs/km: 501.8
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/compare/255-40-r18-vs-225-45-r17Common alternatives to 255/40 R18
These sizes are within a few percent overall diameter — usually safe on the same vehicle from a speedometer-error standpoint.
Compare 255/40 R18 with popular upgrades
Plus-size upgrades from 255/40 R18
Fitment safety note
Overall diameter, speedometer error and circumference are only part of the picture. Physical clearance, wheel offset (ET), hub bore, suspension geometry, load index and your manufacturer's recommendation all decide whether a size is truly safe on your car. Always cross-check with the vehicle manual or a qualified fitter before changing tire size.