Tire Size Box

235/40R20 vs 235/45R20

235/45R20 compared with 235/40R20

+3.4% diameter, 0.0" width

235/45R20 is 28.33" tall vs 27.40". More than the 3% rule of thumb, so check carefully before switching.

27.4"20" rim3.7"9.3" wide
235/40R20 (solid) compared with 235/45R20 (dashed outline), drawn to scale

With 235/45R20 tires, when your speedometer says 60 mph you are really going

62.0 mph

235/45R20 is +0.93 inches (+3.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/40R20235/45R20Difference%
Overall diameter27.40"28.33"+0.93"+3.4%
Section width9.25"9.25"0.00"0.0%
Sidewall height3.70"4.16"+0.46"+12.5%
Circumference86.08"88.99"+2.91"+3.4%
Revolutions per mile736.0712.0−24.0−3.3%

Speedometer difference

If your vehicle came with 235/40R20 and you fit 235/45R20, 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 mph41.4 mph+1.4 mph
50 mph51.7 mph+1.7 mph
60 mph62.0 mph+2.0 mph
65 mph67.2 mph+2.2 mph
70 mph72.4 mph+2.4 mph
75 mph77.5 mph+2.5 mph
80 mph82.7 mph+2.7 mph

Each size on its own

Questions

Which is bigger, 235/40R20 or 235/45R20?
235/45R20 is taller by 0.93 inches (28.33" vs 27.40"). 235/40R20 is the same width.
Can I replace 235/40R20 with 235/45R20?
It is not recommended without checking carefully. The diameter difference is +3.4%, 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 235/45R20 change my speedometer?
When your speedometer reads 60 mph with 235/45R20 fitted (calibrated for 235/40R20), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.4%.
Will 235/45R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.46 inches of ground clearance at the axle.

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