Tire Size Box

295/30R21 vs 295/40R21

295/40R21 compared with 295/30R21

+8.3% diameter, 0.0" width

295/40R21 is 30.29" tall vs 27.97". More than the 3% rule of thumb, so check carefully before switching.

28.0"21" rim3.5"11.6" wide
295/30R21 (solid) compared with 295/40R21 (dashed outline), drawn to scale

With 295/40R21 tires, when your speedometer says 60 mph you are really going

65.0 mph

295/40R21 is +2.32 inches (+8.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

295/30R21295/40R21Difference%
Overall diameter27.97"30.29"+2.32"+8.3%
Section width11.61"11.61"0.00"0.0%
Sidewall height3.48"4.65"+1.16"+33.3%
Circumference87.87"95.16"+7.30"+8.3%
Revolutions per mile721.1665.8−55.3−7.7%

Speedometer difference

If your vehicle came with 295/30R21 and you fit 295/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.7 mph+1.7 mph
30 mph32.5 mph+2.5 mph
40 mph43.3 mph+3.3 mph
50 mph54.2 mph+4.2 mph
60 mph65.0 mph+5.0 mph
65 mph70.4 mph+5.4 mph
70 mph75.8 mph+5.8 mph
75 mph81.2 mph+6.2 mph
80 mph86.6 mph+6.6 mph

Each size on its own

Questions

Which is bigger, 295/30R21 or 295/40R21?
295/40R21 is taller by 2.32 inches (30.29" vs 27.97"). 295/30R21 is the same width.
Can I replace 295/30R21 with 295/40R21?
It is not recommended without checking carefully. The diameter difference is +8.3%, 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 295/40R21 change my speedometer?
When your speedometer reads 60 mph with 295/40R21 fitted (calibrated for 295/30R21), your real speed is 65.0 mph. Your odometer will under-count miles by about 8.3%.
Will 295/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.16 inches of ground clearance at the axle.

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