Tire Size Box

255/35R21 vs 255/40R21

255/40R21 compared with 255/35R21

+3.6% diameter, 0.0" width

255/40R21 is 29.03" tall vs 28.03". More than the 3% rule of thumb, so check carefully before switching.

28.0"21" rim3.5"10.0" wide
255/35R21 (solid) compared with 255/40R21 (dashed outline), drawn to scale

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

62.1 mph

255/40R21 is +1.00 inches (+3.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/35R21255/40R21Difference%
Overall diameter28.03"29.03"+1.00"+3.6%
Section width10.04"10.04"0.00"0.0%
Sidewall height3.51"4.02"+0.50"+14.3%
Circumference88.05"91.21"+3.15"+3.6%
Revolutions per mile719.6694.7−24.9−3.5%

Speedometer difference

If your vehicle came with 255/35R21 and you fit 255/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.1 mph+1.1 mph
40 mph41.4 mph+1.4 mph
50 mph51.8 mph+1.8 mph
60 mph62.1 mph+2.1 mph
65 mph67.3 mph+2.3 mph
70 mph72.5 mph+2.5 mph
75 mph77.7 mph+2.7 mph
80 mph82.9 mph+2.9 mph

Each size on its own

Questions

Which is bigger, 255/35R21 or 255/40R21?
255/40R21 is taller by 1.00 inches (29.03" vs 28.03"). 255/35R21 is the same width.
Can I replace 255/35R21 with 255/40R21?
It is not recommended without checking carefully. The diameter difference is +3.6%, 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 255/35R21), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.6%.
Will 255/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.50 inches of ground clearance at the axle.

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