Tire Size Box

305/40R21 vs 305/45R21

305/45R21 compared with 305/40R21

+3.9% diameter, 0.0" width

305/45R21 is 31.81" tall vs 30.61". More than the 3% rule of thumb, so check carefully before switching.

30.6"21" rim4.8"12.0" wide
305/40R21 (solid) compared with 305/45R21 (dashed outline), drawn to scale

With 305/45R21 tires, when your speedometer says 60 mph you are really going

62.4 mph

305/45R21 is +1.20 inches (+3.9%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/40R21305/45R21Difference%
Overall diameter30.61"31.81"+1.20"+3.9%
Section width12.01"12.01"0.00"0.0%
Sidewall height4.80"5.40"+0.60"+12.5%
Circumference96.15"99.92"+3.77"+3.9%
Revolutions per mile659.0634.1−24.9−3.8%

Speedometer difference

If your vehicle came with 305/40R21 and you fit 305/45R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.2 mph+1.2 mph
40 mph41.6 mph+1.6 mph
50 mph52.0 mph+2.0 mph
60 mph62.4 mph+2.4 mph
65 mph67.6 mph+2.6 mph
70 mph72.7 mph+2.7 mph
75 mph77.9 mph+2.9 mph
80 mph83.1 mph+3.1 mph

Each size on its own

Questions

Which is bigger, 305/40R21 or 305/45R21?
305/45R21 is taller by 1.20 inches (31.81" vs 30.61"). 305/40R21 is the same width.
Can I replace 305/40R21 with 305/45R21?
It is not recommended without checking carefully. The diameter difference is +3.9%, 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 305/45R21 change my speedometer?
When your speedometer reads 60 mph with 305/45R21 fitted (calibrated for 305/40R21), your real speed is 62.4 mph. Your odometer will under-count miles by about 3.9%.
Will 305/45R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.60 inches of ground clearance at the axle.

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