Tire Size Box

235/40R21 vs 235/45R21

235/45R21 compared with 235/40R21

+3.3% diameter, 0.0" width

235/45R21 is 29.33" 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 235/45R21 (dashed outline), drawn to scale

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

62.0 mph

235/45R21 is +0.93 inches (+3.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/40R21235/45R21Difference%
Overall diameter28.40"29.33"+0.93"+3.3%
Section width9.25"9.25"0.00"0.0%
Sidewall height3.70"4.16"+0.46"+12.5%
Circumference89.23"92.13"+2.91"+3.3%
Revolutions per mile710.1687.7−22.4−3.2%

Speedometer difference

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

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.6 mph+1.6 mph
60 mph62.0 mph+2.0 mph
65 mph67.1 mph+2.1 mph
70 mph72.3 mph+2.3 mph
75 mph77.4 mph+2.4 mph
80 mph82.6 mph+2.6 mph

Each size on its own

Questions

Which is bigger, 235/40R21 or 235/45R21?
235/45R21 is taller by 0.93 inches (29.33" vs 28.40"). 235/40R21 is the same width.
Can I replace 235/40R21 with 235/45R21?
It is not recommended without checking carefully. The diameter difference is +3.3%, 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 235/45R21 change my speedometer?
When your speedometer reads 60 mph with 235/45R21 fitted (calibrated for 235/40R21), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.3%.
Will 235/45R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.46 inches of ground clearance at the axle.

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