Tire Size Box

255/30R21 vs 255/40R21

255/40R21 compared with 255/30R21

+7.4% diameter, 0.0" width

255/40R21 is 29.03" tall vs 27.02". More than the 3% rule of thumb, so check carefully before switching.

27.0"21" rim3.0"10.0" wide
255/30R21 (solid) compared with 255/40R21 (dashed outline), drawn to scale

With 255/40R21 tires, when your speedometer says 60 mph you are really going

64.5 mph

255/40R21 is +2.01 inches (+7.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/30R21255/40R21Difference%
Overall diameter27.02"29.03"+2.01"+7.4%
Section width10.04"10.04"0.00"0.0%
Sidewall height3.01"4.02"+1.00"+33.3%
Circumference84.90"91.21"+6.31"+7.4%
Revolutions per mile746.3694.7−51.6−6.9%

Speedometer difference

If your vehicle came with 255/30R21 and you fit 255/40R21, 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 mph69.8 mph+4.8 mph
70 mph75.2 mph+5.2 mph
75 mph80.6 mph+5.6 mph
80 mph85.9 mph+5.9 mph

Each size on its own

Questions

Which is bigger, 255/30R21 or 255/40R21?
255/40R21 is taller by 2.01 inches (29.03" vs 27.02"). 255/30R21 is the same width.
Can I replace 255/30R21 with 255/40R21?
It is not recommended without checking carefully. The diameter difference is +7.4%, 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 255/40R21 change my speedometer?
When your speedometer reads 60 mph with 255/40R21 fitted (calibrated for 255/30R21), your real speed is 64.5 mph. Your odometer will under-count miles by about 7.4%.
Will 255/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.00 inches of ground clearance at the axle.

Other comparisons