Tire Size Box

275/35R21 vs 275/40R21

275/40R21 compared with 275/35R21

+3.8% diameter, 0.0" width

275/40R21 is 29.66" tall vs 28.58". More than the 3% rule of thumb, so check carefully before switching.

28.6"21" rim3.8"10.8" wide
275/35R21 (solid) compared with 275/40R21 (dashed outline), drawn to scale

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

62.3 mph

275/40R21 is +1.08 inches (+3.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

275/35R21275/40R21Difference%
Overall diameter28.58"29.66"+1.08"+3.8%
Section width10.83"10.83"0.00"0.0%
Sidewall height3.79"4.33"+0.54"+14.3%
Circumference89.78"93.18"+3.40"+3.8%
Revolutions per mile705.7679.9−25.8−3.7%

Speedometer difference

If your vehicle came with 275/35R21 and you fit 275/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.1 mph+1.1 mph
40 mph41.5 mph+1.5 mph
50 mph51.9 mph+1.9 mph
60 mph62.3 mph+2.3 mph
65 mph67.5 mph+2.5 mph
70 mph72.7 mph+2.7 mph
75 mph77.8 mph+2.8 mph
80 mph83.0 mph+3.0 mph

Each size on its own

Questions

Which is bigger, 275/35R21 or 275/40R21?
275/40R21 is taller by 1.08 inches (29.66" vs 28.58"). 275/35R21 is the same width.
Can I replace 275/35R21 with 275/40R21?
It is not recommended without checking carefully. The diameter difference is +3.8%, 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 275/35R21), your real speed is 62.3 mph. Your odometer will under-count miles by about 3.8%.
Will 275/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.54 inches of ground clearance at the axle.

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