Tire Size Box

285/40R20 vs 285/45R20

285/45R20 compared with 285/40R20

+3.9% diameter, 0.0" width

285/45R20 is 30.10" tall vs 28.98". More than the 3% rule of thumb, so check carefully before switching.

29.0"20" rim4.5"11.2" wide
285/40R20 (solid) compared with 285/45R20 (dashed outline), drawn to scale

With 285/45R20 tires, when your speedometer says 60 mph you are really going

62.3 mph

285/45R20 is +1.12 inches (+3.9%) in overall diameter. That is more than the 3% most shops and makers recommend.

285/40R20285/45R20Difference%
Overall diameter28.98"30.10"+1.12"+3.9%
Section width11.22"11.22"0.00"0.0%
Sidewall height4.49"5.05"+0.56"+12.5%
Circumference91.03"94.56"+3.53"+3.9%
Revolutions per mile696.0670.1−25.9−3.7%

Speedometer difference

If your vehicle came with 285/40R20 and you fit 285/45R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.2 mph+1.2 mph
40 mph41.5 mph+1.5 mph
50 mph51.9 mph+1.9 mph
60 mph62.3 mph+2.3 mph
65 mph67.5 mph+2.5 mph
70 mph72.7 mph+2.7 mph
75 mph77.9 mph+2.9 mph
80 mph83.1 mph+3.1 mph

Each size on its own

Questions

Which is bigger, 285/40R20 or 285/45R20?
285/45R20 is taller by 1.12 inches (30.10" vs 28.98"). 285/40R20 is the same width.
Can I replace 285/40R20 with 285/45R20?
It is not recommended without checking carefully. The diameter difference is +3.9%, 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 285/45R20 change my speedometer?
When your speedometer reads 60 mph with 285/45R20 fitted (calibrated for 285/40R20), your real speed is 62.3 mph. Your odometer will under-count miles by about 3.9%.
Will 285/45R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.56 inches of ground clearance at the axle.

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