Tire Size Box

225/40R20 vs 225/45R20

225/45R20 compared with 225/40R20

+3.3% diameter, 0.0" width

225/45R20 is 27.97" tall vs 27.09". More than the 3% rule of thumb, so check carefully before switching.

27.1"20" rim3.5"8.9" wide
225/40R20 (solid) compared with 225/45R20 (dashed outline), drawn to scale

With 225/45R20 tires, when your speedometer says 60 mph you are really going

62.0 mph

225/45R20 is +0.89 inches (+3.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

225/40R20225/45R20Difference%
Overall diameter27.09"27.97"+0.89"+3.3%
Section width8.86"8.86"0.00"0.0%
Sidewall height3.54"3.99"+0.44"+12.5%
Circumference85.10"87.88"+2.78"+3.3%
Revolutions per mile744.6721.0−23.6−3.2%

Speedometer difference

If your vehicle came with 225/40R20 and you fit 225/45R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.6 mph+1.6 mph
60 mph62.0 mph+2.0 mph
65 mph67.1 mph+2.1 mph
70 mph72.3 mph+2.3 mph
75 mph77.5 mph+2.5 mph
80 mph82.6 mph+2.6 mph

Each size on its own

Questions

Which is bigger, 225/40R20 or 225/45R20?
225/45R20 is taller by 0.89 inches (27.97" vs 27.09"). 225/40R20 is the same width.
Can I replace 225/40R20 with 225/45R20?
It is not recommended without checking carefully. The diameter difference is +3.3%, 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 225/45R20 change my speedometer?
When your speedometer reads 60 mph with 225/45R20 fitted (calibrated for 225/40R20), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.3%.
Will 225/45R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.44 inches of ground clearance at the axle.

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