Tire Size Box

225/35R18 vs 225/40R18

225/40R18 compared with 225/35R18

+3.7% diameter, 0.0" width

225/40R18 is 25.09" tall vs 24.20". More than the 3% rule of thumb, so check carefully before switching.

24.2"18" rim3.1"8.9" wide
225/35R18 (solid) compared with 225/40R18 (dashed outline), drawn to scale

With 225/40R18 tires, when your speedometer says 60 mph you are really going

62.2 mph

225/40R18 is +0.89 inches (+3.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

225/35R18225/40R18Difference%
Overall diameter24.20"25.09"+0.89"+3.7%
Section width8.86"8.86"0.00"0.0%
Sidewall height3.10"3.54"+0.44"+14.3%
Circumference76.03"78.81"+2.78"+3.7%
Revolutions per mile833.4803.9−29.4−3.5%

Speedometer difference

If your vehicle came with 225/35R18 and you fit 225/40R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.1 mph+1.1 mph
40 mph41.5 mph+1.5 mph
50 mph51.8 mph+1.8 mph
60 mph62.2 mph+2.2 mph
65 mph67.4 mph+2.4 mph
70 mph72.6 mph+2.6 mph
75 mph77.7 mph+2.7 mph
80 mph82.9 mph+2.9 mph

Each size on its own

Questions

Which is bigger, 225/35R18 or 225/40R18?
225/40R18 is taller by 0.89 inches (25.09" vs 24.20"). 225/35R18 is the same width.
Can I replace 225/35R18 with 225/40R18?
It is not recommended without checking carefully. The diameter difference is +3.7%, more than the 3% most shops and makers allow. It will throw off the speedometer and may affect ABS, stability control and clearance.
How much will 225/40R18 change my speedometer?
When your speedometer reads 60 mph with 225/40R18 fitted (calibrated for 225/35R18), your real speed is 62.2 mph. Your odometer will under-count miles by about 3.7%.
Will 225/40R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.44 inches of ground clearance at the axle.

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