Tire Size Box

345/30R20 vs 345/40R20

345/40R20 compared with 345/30R20

+9.7% diameter, 0.0" width

345/40R20 is 30.87" tall vs 28.15". More than the 3% rule of thumb, so check carefully before switching.

28.1"20" rim4.1"13.6" wide
345/30R20 (solid) compared with 345/40R20 (dashed outline), drawn to scale

With 345/40R20 tires, when your speedometer says 60 mph you are really going

65.8 mph

345/40R20 is +2.72 inches (+9.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

345/30R20345/40R20Difference%
Overall diameter28.15"30.87"+2.72"+9.7%
Section width13.58"13.58"0.00"0.0%
Sidewall height4.07"5.43"+1.36"+33.3%
Circumference88.43"96.97"+8.53"+9.7%
Revolutions per mile716.5653.4−63.1−8.8%

Speedometer difference

If your vehicle came with 345/30R20 and you fit 345/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.9 mph+1.9 mph
30 mph32.9 mph+2.9 mph
40 mph43.9 mph+3.9 mph
50 mph54.8 mph+4.8 mph
60 mph65.8 mph+5.8 mph
65 mph71.3 mph+6.3 mph
70 mph76.8 mph+6.8 mph
75 mph82.2 mph+7.2 mph
80 mph87.7 mph+7.7 mph

Each size on its own

Questions

Which is bigger, 345/30R20 or 345/40R20?
345/40R20 is taller by 2.72 inches (30.87" vs 28.15"). 345/30R20 is the same width.
Can I replace 345/30R20 with 345/40R20?
It is not recommended without checking carefully. The diameter difference is +9.7%, 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 345/40R20 change my speedometer?
When your speedometer reads 60 mph with 345/40R20 fitted (calibrated for 345/30R20), your real speed is 65.8 mph. Your odometer will under-count miles by about 9.7%.
Will 345/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.36 inches of ground clearance at the axle.

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