Tire Size Box

225/40R18 vs 245/40R18

245/40R18 compared with 225/40R18

+2.5% diameter, +0.8" width

245/40R18 is 25.72" tall vs 25.09". Within the 3% rule, so usually interchangeable.

25.1"18" rim3.5"8.9" wide
225/40R18 (solid) compared with 245/40R18 (dashed outline), drawn to scale

With 245/40R18 tires, when your speedometer says 60 mph you are really going

61.5 mph

245/40R18 is +0.63 inches (+2.5%) in overall diameter. That is within the 3% most shops and makers allow.

225/40R18245/40R18Difference%
Overall diameter25.09"25.72"+0.63"+2.5%
Section width8.86"9.65"+0.79"+8.9%
Sidewall height3.54"3.86"+0.31"+8.9%
Circumference78.81"80.79"+1.98"+2.5%
Revolutions per mile803.9784.2−19.7−2.4%

Speedometer difference

If your vehicle came with 225/40R18 and you fit 245/40R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.5 mph+0.5 mph
30 mph30.8 mph+0.8 mph
40 mph41.0 mph+1.0 mph
50 mph51.3 mph+1.3 mph
60 mph61.5 mph+1.5 mph
65 mph66.6 mph+1.6 mph
70 mph71.8 mph+1.8 mph
75 mph76.9 mph+1.9 mph
80 mph82.0 mph+2.0 mph

Each size on its own

Questions

Which is bigger, 225/40R18 or 245/40R18?
245/40R18 is taller by 0.63 inches (25.72" vs 25.09"). 245/40R18 is wider by 0.79 inches.
Can I replace 225/40R18 with 245/40R18?
Usually, yes. The diameter difference is +2.5%, inside the 3% rule of thumb most tire shops use. Check that the wider or taller tire clears your suspension and fenders, and that its load index meets your placard.
How much will 245/40R18 change my speedometer?
When your speedometer reads 60 mph with 245/40R18 fitted (calibrated for 225/40R18), your real speed is 61.5 mph. Your odometer will under-count miles by about 2.5%.
Will 245/40R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.31 inches of ground clearance at the axle.

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