Tire Size Box

275/40R21 vs 275/45R21

275/45R21 compared with 275/40R21

+3.7% diameter, 0.0" width

275/45R21 is 30.74" tall vs 29.66". More than the 3% rule of thumb, so check carefully before switching.

29.7"21" rim4.3"10.8" wide
275/40R21 (solid) compared with 275/45R21 (dashed outline), drawn to scale

With 275/45R21 tires, when your speedometer says 60 mph you are really going

62.2 mph

275/45R21 is +1.08 inches (+3.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

275/40R21275/45R21Difference%
Overall diameter29.66"30.74"+1.08"+3.7%
Section width10.83"10.83"0.00"0.0%
Sidewall height4.33"4.87"+0.54"+12.5%
Circumference93.18"96.59"+3.40"+3.7%
Revolutions per mile679.9656.0−23.9−3.5%

Speedometer difference

If your vehicle came with 275/40R21 and you fit 275/45R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.1 mph+1.1 mph
40 mph41.5 mph+1.5 mph
50 mph51.8 mph+1.8 mph
60 mph62.2 mph+2.2 mph
65 mph67.4 mph+2.4 mph
70 mph72.6 mph+2.6 mph
75 mph77.7 mph+2.7 mph
80 mph82.9 mph+2.9 mph

Each size on its own

Questions

Which is bigger, 275/40R21 or 275/45R21?
275/45R21 is taller by 1.08 inches (30.74" vs 29.66"). 275/40R21 is the same width.
Can I replace 275/40R21 with 275/45R21?
It is not recommended without checking carefully. The diameter difference is +3.7%, 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 275/45R21 change my speedometer?
When your speedometer reads 60 mph with 275/45R21 fitted (calibrated for 275/40R21), your real speed is 62.2 mph. Your odometer will under-count miles by about 3.7%.
Will 275/45R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.54 inches of ground clearance at the axle.

Other comparisons