Tire Size Box

245/40R18 vs 245/45R18

245/45R18 compared with 245/40R18

+3.8% diameter, 0.0" width

245/45R18 is 26.68" tall vs 25.72". More than the 3% rule of thumb, so check carefully before switching.

25.7"18" rim3.9"9.6" wide
245/40R18 (solid) compared with 245/45R18 (dashed outline), drawn to scale

With 245/45R18 tires, when your speedometer says 60 mph you are really going

62.3 mph

245/45R18 is +0.96 inches (+3.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

245/40R18245/45R18Difference%
Overall diameter25.72"26.68"+0.96"+3.8%
Section width9.65"9.65"0.00"0.0%
Sidewall height3.86"4.34"+0.48"+12.5%
Circumference80.79"83.82"+3.03"+3.8%
Revolutions per mile784.2755.9−28.4−3.6%

Speedometer difference

If your vehicle came with 245/40R18 and you fit 245/45R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.1 mph+1.1 mph
40 mph41.5 mph+1.5 mph
50 mph51.9 mph+1.9 mph
60 mph62.3 mph+2.3 mph
65 mph67.4 mph+2.4 mph
70 mph72.6 mph+2.6 mph
75 mph77.8 mph+2.8 mph
80 mph83.0 mph+3.0 mph

Each size on its own

Questions

Which is bigger, 245/40R18 or 245/45R18?
245/45R18 is taller by 0.96 inches (26.68" vs 25.72"). 245/40R18 is the same width.
Can I replace 245/40R18 with 245/45R18?
It is not recommended without checking carefully. The diameter difference is +3.8%, 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 245/45R18 change my speedometer?
When your speedometer reads 60 mph with 245/45R18 fitted (calibrated for 245/40R18), your real speed is 62.3 mph. Your odometer will under-count miles by about 3.8%.
Will 245/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.48 inches of ground clearance at the axle.

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