Tire Size Box

245/30R21 vs 245/40R21

245/40R21 compared with 245/30R21

+7.2% diameter, 0.0" width

245/40R21 is 28.72" tall vs 26.79". More than the 3% rule of thumb, so check carefully before switching.

26.8"21" rim2.9"9.6" wide
245/30R21 (solid) compared with 245/40R21 (dashed outline), drawn to scale

With 245/40R21 tires, when your speedometer says 60 mph you are really going

64.3 mph

245/40R21 is +1.93 inches (+7.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

245/30R21245/40R21Difference%
Overall diameter26.79"28.72"+1.93"+7.2%
Section width9.65"9.65"0.00"0.0%
Sidewall height2.89"3.86"+0.96"+33.3%
Circumference84.16"90.22"+6.06"+7.2%
Revolutions per mile752.9702.3−50.6−6.7%

Speedometer difference

If your vehicle came with 245/30R21 and you fit 245/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.4 mph+1.4 mph
30 mph32.2 mph+2.2 mph
40 mph42.9 mph+2.9 mph
50 mph53.6 mph+3.6 mph
60 mph64.3 mph+4.3 mph
65 mph69.7 mph+4.7 mph
70 mph75.0 mph+5.0 mph
75 mph80.4 mph+5.4 mph
80 mph85.8 mph+5.8 mph

Each size on its own

Questions

Which is bigger, 245/30R21 or 245/40R21?
245/40R21 is taller by 1.93 inches (28.72" vs 26.79"). 245/30R21 is the same width.
Can I replace 245/30R21 with 245/40R21?
It is not recommended without checking carefully. The diameter difference is +7.2%, 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 245/40R21 change my speedometer?
When your speedometer reads 60 mph with 245/40R21 fitted (calibrated for 245/30R21), your real speed is 64.3 mph. Your odometer will under-count miles by about 7.2%.
Will 245/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.96 inches of ground clearance at the axle.

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