Tire Size Box

245/30R20 vs 245/40R20

245/40R20 compared with 245/30R20

+7.5% diameter, 0.0" width

245/40R20 is 27.72" tall vs 25.79". More than the 3% rule of thumb, so check carefully before switching.

25.8"20" rim2.9"9.6" wide
245/30R20 (solid) compared with 245/40R20 (dashed outline), drawn to scale

With 245/40R20 tires, when your speedometer says 60 mph you are really going

64.5 mph

245/40R20 is +1.93 inches (+7.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

245/30R20245/40R20Difference%
Overall diameter25.79"27.72"+1.93"+7.5%
Section width9.65"9.65"0.00"0.0%
Sidewall height2.89"3.86"+0.96"+33.3%
Circumference81.01"87.07"+6.06"+7.5%
Revolutions per mile782.1727.7−54.4−7.0%

Speedometer difference

If your vehicle came with 245/30R20 and you fit 245/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.5 mph+1.5 mph
30 mph32.2 mph+2.2 mph
40 mph43.0 mph+3.0 mph
50 mph53.7 mph+3.7 mph
60 mph64.5 mph+4.5 mph
65 mph69.9 mph+4.9 mph
70 mph75.2 mph+5.2 mph
75 mph80.6 mph+5.6 mph
80 mph86.0 mph+6.0 mph

Each size on its own

Questions

Which is bigger, 245/30R20 or 245/40R20?
245/40R20 is taller by 1.93 inches (27.72" vs 25.79"). 245/30R20 is the same width.
Can I replace 245/30R20 with 245/40R20?
It is not recommended without checking carefully. The diameter difference is +7.5%, 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 245/40R20 change my speedometer?
When your speedometer reads 60 mph with 245/40R20 fitted (calibrated for 245/30R20), your real speed is 64.5 mph. Your odometer will under-count miles by about 7.5%.
Will 245/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.96 inches of ground clearance at the axle.

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