Tire Size Box

265/30R20 vs 265/40R20

265/40R20 compared with 265/30R20

+7.9% diameter, 0.0" width

265/40R20 is 28.35" tall vs 26.26". More than the 3% rule of thumb, so check carefully before switching.

26.3"20" rim3.1"10.4" wide
265/30R20 (solid) compared with 265/40R20 (dashed outline), drawn to scale

With 265/40R20 tires, when your speedometer says 60 mph you are really going

64.8 mph

265/40R20 is +2.09 inches (+7.9%) in overall diameter. That is more than the 3% most shops and makers recommend.

265/30R20265/40R20Difference%
Overall diameter26.26"28.35"+2.09"+7.9%
Section width10.43"10.43"0.00"0.0%
Sidewall height3.13"4.17"+1.04"+33.3%
Circumference82.50"89.05"+6.56"+7.9%
Revolutions per mile768.0711.5−56.5−7.4%

Speedometer difference

If your vehicle came with 265/30R20 and you fit 265/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.6 mph+1.6 mph
30 mph32.4 mph+2.4 mph
40 mph43.2 mph+3.2 mph
50 mph54.0 mph+4.0 mph
60 mph64.8 mph+4.8 mph
65 mph70.2 mph+5.2 mph
70 mph75.6 mph+5.6 mph
75 mph81.0 mph+6.0 mph
80 mph86.4 mph+6.4 mph

Each size on its own

Questions

Which is bigger, 265/30R20 or 265/40R20?
265/40R20 is taller by 2.09 inches (28.35" vs 26.26"). 265/30R20 is the same width.
Can I replace 265/30R20 with 265/40R20?
It is not recommended without checking carefully. The diameter difference is +7.9%, 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 265/40R20 change my speedometer?
When your speedometer reads 60 mph with 265/40R20 fitted (calibrated for 265/30R20), your real speed is 64.8 mph. Your odometer will under-count miles by about 7.9%.
Will 265/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.04 inches of ground clearance at the axle.

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