Tire Size Box

305/30R22 vs 305/40R22

305/40R22 compared with 305/30R22

+8.2% diameter, 0.0" width

305/40R22 is 31.61" tall vs 29.20". More than the 3% rule of thumb, so check carefully before switching.

29.2"22" rim3.6"12.0" wide
305/30R22 (solid) compared with 305/40R22 (dashed outline), drawn to scale

With 305/40R22 tires, when your speedometer says 60 mph you are really going

64.9 mph

305/40R22 is +2.40 inches (+8.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/30R22305/40R22Difference%
Overall diameter29.20"31.61"+2.40"+8.2%
Section width12.01"12.01"0.00"0.0%
Sidewall height3.60"4.80"+1.20"+33.3%
Circumference91.75"99.29"+7.54"+8.2%
Revolutions per mile690.6638.1−52.5−7.6%

Speedometer difference

If your vehicle came with 305/30R22 and you fit 305/40R22, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.6 mph+1.6 mph
30 mph32.5 mph+2.5 mph
40 mph43.3 mph+3.3 mph
50 mph54.1 mph+4.1 mph
60 mph64.9 mph+4.9 mph
65 mph70.3 mph+5.3 mph
70 mph75.8 mph+5.8 mph
75 mph81.2 mph+6.2 mph
80 mph86.6 mph+6.6 mph

Each size on its own

Questions

Which is bigger, 305/30R22 or 305/40R22?
305/40R22 is taller by 2.40 inches (31.61" vs 29.20"). 305/30R22 is the same width.
Can I replace 305/30R22 with 305/40R22?
It is not recommended without checking carefully. The diameter difference is +8.2%, 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 305/40R22 change my speedometer?
When your speedometer reads 60 mph with 305/40R22 fitted (calibrated for 305/30R22), your real speed is 64.9 mph. Your odometer will under-count miles by about 8.2%.
Will 305/40R22 lift my vehicle?
Ride height changes by half the diameter difference: +1.20 inches of ground clearance at the axle.

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