Tire Size Box

225/30R20 vs 225/40R20

225/40R20 compared with 225/30R20

+7.0% diameter, 0.0" width

225/40R20 is 27.09" tall vs 25.31". More than the 3% rule of thumb, so check carefully before switching.

25.3"20" rim2.7"8.9" wide
225/30R20 (solid) compared with 225/40R20 (dashed outline), drawn to scale

With 225/40R20 tires, when your speedometer says 60 mph you are really going

64.2 mph

225/40R20 is +1.77 inches (+7.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

225/30R20225/40R20Difference%
Overall diameter25.31"27.09"+1.77"+7.0%
Section width8.86"8.86"0.00"0.0%
Sidewall height2.66"3.54"+0.89"+33.3%
Circumference79.53"85.10"+5.57"+7.0%
Revolutions per mile796.7744.6−52.1−6.5%

Speedometer difference

If your vehicle came with 225/30R20 and you fit 225/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.4 mph+1.4 mph
30 mph32.1 mph+2.1 mph
40 mph42.8 mph+2.8 mph
50 mph53.5 mph+3.5 mph
60 mph64.2 mph+4.2 mph
65 mph69.5 mph+4.5 mph
70 mph74.9 mph+4.9 mph
75 mph80.2 mph+5.2 mph
80 mph85.6 mph+5.6 mph

Each size on its own

Questions

Which is bigger, 225/30R20 or 225/40R20?
225/40R20 is taller by 1.77 inches (27.09" vs 25.31"). 225/30R20 is the same width.
Can I replace 225/30R20 with 225/40R20?
It is not recommended without checking carefully. The diameter difference is +7.0%, 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 225/40R20 change my speedometer?
When your speedometer reads 60 mph with 225/40R20 fitted (calibrated for 225/30R20), your real speed is 64.2 mph. Your odometer will under-count miles by about 7.0%.
Will 225/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.89 inches of ground clearance at the axle.

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