Tire Size Box

305/30R21 vs 305/40R21

305/40R21 compared with 305/30R21

+8.5% diameter, 0.0" width

305/40R21 is 30.61" tall vs 28.20". More than the 3% rule of thumb, so check carefully before switching.

28.2"21" rim3.6"12.0" wide
305/30R21 (solid) compared with 305/40R21 (dashed outline), drawn to scale

With 305/40R21 tires, when your speedometer says 60 mph you are really going

65.1 mph

305/40R21 is +2.40 inches (+8.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/30R21305/40R21Difference%
Overall diameter28.20"30.61"+2.40"+8.5%
Section width12.01"12.01"0.00"0.0%
Sidewall height3.60"4.80"+1.20"+33.3%
Circumference88.61"96.15"+7.54"+8.5%
Revolutions per mile715.1659.0−56.1−7.8%

Speedometer difference

If your vehicle came with 305/30R21 and you fit 305/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.7 mph+1.7 mph
30 mph32.6 mph+2.6 mph
40 mph43.4 mph+3.4 mph
50 mph54.3 mph+4.3 mph
60 mph65.1 mph+5.1 mph
65 mph70.5 mph+5.5 mph
70 mph76.0 mph+6.0 mph
75 mph81.4 mph+6.4 mph
80 mph86.8 mph+6.8 mph

Each size on its own

Questions

Which is bigger, 305/30R21 or 305/40R21?
305/40R21 is taller by 2.40 inches (30.61" vs 28.20"). 305/30R21 is the same width.
Can I replace 305/30R21 with 305/40R21?
It is not recommended without checking carefully. The diameter difference is +8.5%, 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 305/40R21 change my speedometer?
When your speedometer reads 60 mph with 305/40R21 fitted (calibrated for 305/30R21), your real speed is 65.1 mph. Your odometer will under-count miles by about 8.5%.
Will 305/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.20 inches of ground clearance at the axle.

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