Tire Size Box

305/35R20 vs 305/45R20

305/45R20 compared with 305/35R20

+8.5% diameter, 0.0" width

305/45R20 is 30.81" tall vs 28.41". More than the 3% rule of thumb, so check carefully before switching.

28.4"20" rim4.2"12.0" wide
305/35R20 (solid) compared with 305/45R20 (dashed outline), drawn to scale

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

65.1 mph

305/45R20 is +2.40 inches (+8.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/35R20305/45R20Difference%
Overall diameter28.41"30.81"+2.40"+8.5%
Section width12.01"12.01"0.00"0.0%
Sidewall height4.20"5.40"+1.20"+28.6%
Circumference89.24"96.78"+7.54"+8.5%
Revolutions per mile710.0654.7−55.3−7.8%

Speedometer difference

If your vehicle came with 305/35R20 and you fit 305/45R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.7 mph+1.7 mph
30 mph32.5 mph+2.5 mph
40 mph43.4 mph+3.4 mph
50 mph54.2 mph+4.2 mph
60 mph65.1 mph+5.1 mph
65 mph70.5 mph+5.5 mph
70 mph75.9 mph+5.9 mph
75 mph81.3 mph+6.3 mph
80 mph86.8 mph+6.8 mph

Each size on its own

Questions

Which is bigger, 305/35R20 or 305/45R20?
305/45R20 is taller by 2.40 inches (30.81" vs 28.41"). 305/35R20 is the same width.
Can I replace 305/35R20 with 305/45R20?
It is not recommended without checking carefully. The diameter difference is +8.5%, 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 305/35R20), your real speed is 65.1 mph. Your odometer will under-count miles by about 8.5%.
Will 305/45R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.20 inches of ground clearance at the axle.

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