Tire Size Box

275/30R21 vs 275/40R21

275/40R21 compared with 275/30R21

+7.9% diameter, 0.0" width

275/40R21 is 29.66" tall vs 27.50". More than the 3% rule of thumb, so check carefully before switching.

27.5"21" rim3.2"10.8" wide
275/30R21 (solid) compared with 275/40R21 (dashed outline), drawn to scale

With 275/40R21 tires, when your speedometer says 60 mph you are really going

64.7 mph

275/40R21 is +2.17 inches (+7.9%) in overall diameter. That is more than the 3% most shops and makers recommend.

275/30R21275/40R21Difference%
Overall diameter27.50"29.66"+2.17"+7.9%
Section width10.83"10.83"0.00"0.0%
Sidewall height3.25"4.33"+1.08"+33.3%
Circumference86.38"93.18"+6.80"+7.9%
Revolutions per mile733.5679.9−53.5−7.3%

Speedometer difference

If your vehicle came with 275/30R21 and you fit 275/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.6 mph+1.6 mph
30 mph32.4 mph+2.4 mph
40 mph43.2 mph+3.2 mph
50 mph53.9 mph+3.9 mph
60 mph64.7 mph+4.7 mph
65 mph70.1 mph+5.1 mph
70 mph75.5 mph+5.5 mph
75 mph80.9 mph+5.9 mph
80 mph86.3 mph+6.3 mph

Each size on its own

Questions

Which is bigger, 275/30R21 or 275/40R21?
275/40R21 is taller by 2.17 inches (29.66" vs 27.50"). 275/30R21 is the same width.
Can I replace 275/30R21 with 275/40R21?
It is not recommended without checking carefully. The diameter difference is +7.9%, 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 275/40R21 change my speedometer?
When your speedometer reads 60 mph with 275/40R21 fitted (calibrated for 275/30R21), your real speed is 64.7 mph. Your odometer will under-count miles by about 7.9%.
Will 275/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.08 inches of ground clearance at the axle.

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