Tire Size Box

235/40R18 vs 225/45R18

225/45R18 compared with 235/40R18

+2.2% diameter, −0.4" width

225/45R18 is 25.97" tall vs 25.40". Within the 3% rule, so usually interchangeable.

25.4"18" rim3.7"9.3" wide
235/40R18 (solid) compared with 225/45R18 (dashed outline), drawn to scale

With 225/45R18 tires, when your speedometer says 60 mph you are really going

61.3 mph

225/45R18 is +0.57 inches (+2.2%) in overall diameter. That is within the 3% most shops and makers allow.

235/40R18225/45R18Difference%
Overall diameter25.40"25.97"+0.57"+2.2%
Section width9.25"8.86"−0.39"−4.3%
Sidewall height3.70"3.99"+0.29"+7.7%
Circumference79.80"81.59"+1.79"+2.2%
Revolutions per mile794.0776.5−17.5−2.2%

Speedometer difference

If your vehicle came with 235/40R18 and you fit 225/45R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.4 mph+0.4 mph
30 mph30.7 mph+0.7 mph
40 mph40.9 mph+0.9 mph
50 mph51.1 mph+1.1 mph
60 mph61.3 mph+1.3 mph
65 mph66.5 mph+1.5 mph
70 mph71.6 mph+1.6 mph
75 mph76.7 mph+1.7 mph
80 mph81.8 mph+1.8 mph

Each size on its own

Questions

Which is bigger, 235/40R18 or 225/45R18?
225/45R18 is taller by 0.57 inches (25.97" vs 25.40"). 235/40R18 is wider by 0.39 inches.
Can I replace 235/40R18 with 225/45R18?
Usually, yes. The diameter difference is +2.2%, 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 225/45R18 change my speedometer?
When your speedometer reads 60 mph with 225/45R18 fitted (calibrated for 235/40R18), your real speed is 61.3 mph. Your odometer will under-count miles by about 2.2%.
Will 225/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.29 inches of ground clearance at the axle.

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