Tire Size Box

245/40R21 vs 245/45R21

245/45R21 compared with 245/40R21

+3.4% diameter, 0.0" width

245/45R21 is 29.68" tall vs 28.72". More than the 3% rule of thumb, so check carefully before switching.

28.7"21" rim3.9"9.6" wide
245/40R21 (solid) compared with 245/45R21 (dashed outline), drawn to scale

With 245/45R21 tires, when your speedometer says 60 mph you are really going

62.0 mph

245/45R21 is +0.96 inches (+3.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

245/40R21245/45R21Difference%
Overall diameter28.72"29.68"+0.96"+3.4%
Section width9.65"9.65"0.00"0.0%
Sidewall height3.86"4.34"+0.48"+12.5%
Circumference90.22"93.25"+3.03"+3.4%
Revolutions per mile702.3679.5−22.8−3.2%

Speedometer difference

If your vehicle came with 245/40R21 and you fit 245/45R21, 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 mph51.7 mph+1.7 mph
60 mph62.0 mph+2.0 mph
65 mph67.2 mph+2.2 mph
70 mph72.4 mph+2.4 mph
75 mph77.5 mph+2.5 mph
80 mph82.7 mph+2.7 mph

Each size on its own

Questions

Which is bigger, 245/40R21 or 245/45R21?
245/45R21 is taller by 0.96 inches (29.68" vs 28.72"). 245/40R21 is the same width.
Can I replace 245/40R21 with 245/45R21?
It is not recommended without checking carefully. The diameter difference is +3.4%, 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 245/45R21 change my speedometer?
When your speedometer reads 60 mph with 245/45R21 fitted (calibrated for 245/40R21), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.4%.
Will 245/45R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.48 inches of ground clearance at the axle.

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