Tire Size Box

305/40R18 vs 305/45R18

305/45R18 compared with 305/40R18

+4.3% diameter, 0.0" width

305/45R18 is 28.81" tall vs 27.61". More than the 3% rule of thumb, so check carefully before switching.

27.6"18" rim4.8"12.0" wide
305/40R18 (solid) compared with 305/45R18 (dashed outline), drawn to scale

With 305/45R18 tires, when your speedometer says 60 mph you are really going

62.6 mph

305/45R18 is +1.20 inches (+4.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/40R18305/45R18Difference%
Overall diameter27.61"28.81"+1.20"+4.3%
Section width12.01"12.01"0.00"0.0%
Sidewall height4.80"5.40"+0.60"+12.5%
Circumference86.73"90.50"+3.77"+4.3%
Revolutions per mile730.6700.1−30.5−4.2%

Speedometer difference

If your vehicle came with 305/40R18 and you fit 305/45R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.3 mph+1.3 mph
40 mph41.7 mph+1.7 mph
50 mph52.2 mph+2.2 mph
60 mph62.6 mph+2.6 mph
65 mph67.8 mph+2.8 mph
70 mph73.0 mph+3.0 mph
75 mph78.3 mph+3.3 mph
80 mph83.5 mph+3.5 mph

Each size on its own

Questions

Which is bigger, 305/40R18 or 305/45R18?
305/45R18 is taller by 1.20 inches (28.81" vs 27.61"). 305/40R18 is the same width.
Can I replace 305/40R18 with 305/45R18?
It is not recommended without checking carefully. The diameter difference is +4.3%, 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 305/45R18 change my speedometer?
When your speedometer reads 60 mph with 305/45R18 fitted (calibrated for 305/40R18), your real speed is 62.6 mph. Your odometer will under-count miles by about 4.3%.
Will 305/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.60 inches of ground clearance at the axle.

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