Tire Size Box

305/40R20 vs 305/45R20

305/45R20 compared with 305/40R20

+4.1% diameter, 0.0" width

305/45R20 is 30.81" tall vs 29.61". More than the 3% rule of thumb, so check carefully before switching.

29.6"20" rim4.8"12.0" wide
305/40R20 (solid) compared with 305/45R20 (dashed outline), drawn to scale

With 305/45R20 tires, when your speedometer says 60 mph you are really going

62.4 mph

305/45R20 is +1.20 inches (+4.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/40R20305/45R20Difference%
Overall diameter29.61"30.81"+1.20"+4.1%
Section width12.01"12.01"0.00"0.0%
Sidewall height4.80"5.40"+0.60"+12.5%
Circumference93.01"96.78"+3.77"+4.1%
Revolutions per mile681.2654.7−26.6−3.9%

Speedometer difference

If your vehicle came with 305/40R20 and you fit 305/45R20, 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 mph72.8 mph+2.8 mph
75 mph78.0 mph+3.0 mph
80 mph83.2 mph+3.2 mph

Each size on its own

Questions

Which is bigger, 305/40R20 or 305/45R20?
305/45R20 is taller by 1.20 inches (30.81" vs 29.61"). 305/40R20 is the same width.
Can I replace 305/40R20 with 305/45R20?
It is not recommended without checking carefully. The diameter difference is +4.1%, 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 305/45R20 change my speedometer?
When your speedometer reads 60 mph with 305/45R20 fitted (calibrated for 305/40R20), your real speed is 62.4 mph. Your odometer will under-count miles by about 4.1%.
Will 305/45R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.60 inches of ground clearance at the axle.

Other comparisons