Tire Size Box

235/40R21 vs 235/50R21

235/50R21 compared with 235/40R21

+6.5% diameter, 0.0" width

235/50R21 is 30.25" tall vs 28.40". More than the 3% rule of thumb, so check carefully before switching.

28.4"21" rim3.7"9.3" wide
235/40R21 (solid) compared with 235/50R21 (dashed outline), drawn to scale

With 235/50R21 tires, when your speedometer says 60 mph you are really going

63.9 mph

235/50R21 is +1.85 inches (+6.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/40R21235/50R21Difference%
Overall diameter28.40"30.25"+1.85"+6.5%
Section width9.25"9.25"0.00"0.0%
Sidewall height3.70"4.63"+0.93"+25.0%
Circumference89.23"95.04"+5.81"+6.5%
Revolutions per mile710.1666.7−43.4−6.1%

Speedometer difference

If your vehicle came with 235/40R21 and you fit 235/50R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.3 mph+1.3 mph
30 mph32.0 mph+2.0 mph
40 mph42.6 mph+2.6 mph
50 mph53.3 mph+3.3 mph
60 mph63.9 mph+3.9 mph
65 mph69.2 mph+4.2 mph
70 mph74.6 mph+4.6 mph
75 mph79.9 mph+4.9 mph
80 mph85.2 mph+5.2 mph

Each size on its own

Questions

Which is bigger, 235/40R21 or 235/50R21?
235/50R21 is taller by 1.85 inches (30.25" vs 28.40"). 235/40R21 is the same width.
Can I replace 235/40R21 with 235/50R21?
It is not recommended without checking carefully. The diameter difference is +6.5%, 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 235/50R21 change my speedometer?
When your speedometer reads 60 mph with 235/50R21 fitted (calibrated for 235/40R21), your real speed is 63.9 mph. Your odometer will under-count miles by about 6.5%.
Will 235/50R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.93 inches of ground clearance at the axle.

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