Tire Size Box

235/40R18 vs 235/45R18

235/45R18 compared with 235/40R18

+3.6% diameter, 0.0" width

235/45R18 is 26.33" tall vs 25.40". More than the 3% rule of thumb, so check carefully before switching.

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

With 235/45R18 tires, when your speedometer says 60 mph you are really going

62.2 mph

235/45R18 is +0.93 inches (+3.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/40R18235/45R18Difference%
Overall diameter25.40"26.33"+0.93"+3.6%
Section width9.25"9.25"0.00"0.0%
Sidewall height3.70"4.16"+0.46"+12.5%
Circumference79.80"82.71"+2.91"+3.6%
Revolutions per mile794.0766.1−27.9−3.5%

Speedometer difference

If your vehicle came with 235/40R18 and you fit 235/45R18, 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 mph72.5 mph+2.5 mph
75 mph77.7 mph+2.7 mph
80 mph82.9 mph+2.9 mph

Each size on its own

Questions

Which is bigger, 235/40R18 or 235/45R18?
235/45R18 is taller by 0.93 inches (26.33" vs 25.40"). 235/40R18 is the same width.
Can I replace 235/40R18 with 235/45R18?
It is not recommended without checking carefully. The diameter difference is +3.6%, 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 235/45R18 change my speedometer?
When your speedometer reads 60 mph with 235/45R18 fitted (calibrated for 235/40R18), your real speed is 62.2 mph. Your odometer will under-count miles by about 3.6%.
Will 235/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.46 inches of ground clearance at the axle.

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