Tire Size Box

265/40R21 vs 265/45R21

265/45R21 compared with 265/40R21

+3.6% diameter, 0.0" width

265/45R21 is 30.39" tall vs 29.35". More than the 3% rule of thumb, so check carefully before switching.

29.3"21" rim4.2"10.4" wide
265/40R21 (solid) compared with 265/45R21 (dashed outline), drawn to scale

With 265/45R21 tires, when your speedometer says 60 mph you are really going

62.1 mph

265/45R21 is +1.04 inches (+3.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

265/40R21265/45R21Difference%
Overall diameter29.35"30.39"+1.04"+3.6%
Section width10.43"10.43"0.00"0.0%
Sidewall height4.17"4.69"+0.52"+12.5%
Circumference92.19"95.47"+3.28"+3.6%
Revolutions per mile687.2663.6−23.6−3.4%

Speedometer difference

If your vehicle came with 265/40R21 and you fit 265/45R21, 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 mph41.4 mph+1.4 mph
50 mph51.8 mph+1.8 mph
60 mph62.1 mph+2.1 mph
65 mph67.3 mph+2.3 mph
70 mph72.5 mph+2.5 mph
75 mph77.7 mph+2.7 mph
80 mph82.8 mph+2.8 mph

Each size on its own

Questions

Which is bigger, 265/40R21 or 265/45R21?
265/45R21 is taller by 1.04 inches (30.39" vs 29.35"). 265/40R21 is the same width.
Can I replace 265/40R21 with 265/45R21?
It is not recommended without checking carefully. The diameter difference is +3.6%, 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 265/45R21 change my speedometer?
When your speedometer reads 60 mph with 265/45R21 fitted (calibrated for 265/40R21), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.6%.
Will 265/45R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.52 inches of ground clearance at the axle.

Other comparisons