Tire Size Box

345/30R21 vs 345/40R21

345/40R21 compared with 345/30R21

+9.3% diameter, 0.0" width

345/40R21 is 31.87" tall vs 29.15". More than the 3% rule of thumb, so check carefully before switching.

29.1"21" rim4.1"13.6" wide
345/30R21 (solid) compared with 345/40R21 (dashed outline), drawn to scale

With 345/40R21 tires, when your speedometer says 60 mph you are really going

65.6 mph

345/40R21 is +2.72 inches (+9.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

345/30R21345/40R21Difference%
Overall diameter29.15"31.87"+2.72"+9.3%
Section width13.58"13.58"0.00"0.0%
Sidewall height4.07"5.43"+1.36"+33.3%
Circumference91.58"100.11"+8.53"+9.3%
Revolutions per mile691.9632.9−59.0−8.5%

Speedometer difference

If your vehicle came with 345/30R21 and you fit 345/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.9 mph+1.9 mph
30 mph32.8 mph+2.8 mph
40 mph43.7 mph+3.7 mph
50 mph54.7 mph+4.7 mph
60 mph65.6 mph+5.6 mph
65 mph71.1 mph+6.1 mph
70 mph76.5 mph+6.5 mph
75 mph82.0 mph+7.0 mph
80 mph87.5 mph+7.5 mph

Each size on its own

Questions

Which is bigger, 345/30R21 or 345/40R21?
345/40R21 is taller by 2.72 inches (31.87" vs 29.15"). 345/30R21 is the same width.
Can I replace 345/30R21 with 345/40R21?
It is not recommended without checking carefully. The diameter difference is +9.3%, 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 345/40R21 change my speedometer?
When your speedometer reads 60 mph with 345/40R21 fitted (calibrated for 345/30R21), your real speed is 65.6 mph. Your odometer will under-count miles by about 9.3%.
Will 345/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.36 inches of ground clearance at the axle.

Other comparisons