Tire Size Box

255/40R21 vs 235/45R21

235/45R21 compared with 255/40R21

+1.0% diameter, −0.8" width

235/45R21 is 29.33" tall vs 29.03". Within the 3% rule, so usually interchangeable.

29.0"21" rim4.0"10.0" wide
255/40R21 (solid) compared with 235/45R21 (dashed outline), drawn to scale

With 235/45R21 tires, when your speedometer says 60 mph you are really going

60.6 mph

235/45R21 is +0.30 inches (+1.0%) in overall diameter. That is within the 3% most shops and makers allow.

255/40R21235/45R21Difference%
Overall diameter29.03"29.33"+0.30"+1.0%
Section width10.04"9.25"−0.79"−7.8%
Sidewall height4.02"4.16"+0.15"+3.7%
Circumference91.21"92.13"+0.93"+1.0%
Revolutions per mile694.7687.7−7.0−1.0%

Speedometer difference

If your vehicle came with 255/40R21 and you fit 235/45R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.2 mph+0.2 mph
30 mph30.3 mph+0.3 mph
40 mph40.4 mph+0.4 mph
50 mph50.5 mph+0.5 mph
60 mph60.6 mph+0.6 mph
65 mph65.7 mph+0.7 mph
70 mph70.7 mph+0.7 mph
75 mph75.8 mph+0.8 mph
80 mph80.8 mph+0.8 mph

Each size on its own

Questions

Which is bigger, 255/40R21 or 235/45R21?
235/45R21 is taller by 0.30 inches (29.33" vs 29.03"). 255/40R21 is wider by 0.79 inches.
Can I replace 255/40R21 with 235/45R21?
Usually, yes. The diameter difference is +1.0%, inside the 3% rule of thumb most tire shops use. Check that the wider or taller tire clears your suspension and fenders, and that its load index meets your placard.
How much will 235/45R21 change my speedometer?
When your speedometer reads 60 mph with 235/45R21 fitted (calibrated for 255/40R21), your real speed is 60.6 mph. Your odometer will under-count miles by about 1.0%.
Will 235/45R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.15 inches of ground clearance at the axle.

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