Tire Size Box

305/35R21 vs 305/45R21

305/45R21 compared with 305/35R21

+8.2% diameter, 0.0" width

305/45R21 is 31.81" tall vs 29.41". More than the 3% rule of thumb, so check carefully before switching.

29.4"21" rim4.2"12.0" wide
305/35R21 (solid) compared with 305/45R21 (dashed outline), drawn to scale

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

64.9 mph

305/45R21 is +2.40 inches (+8.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/35R21305/45R21Difference%
Overall diameter29.41"31.81"+2.40"+8.2%
Section width12.01"12.01"0.00"0.0%
Sidewall height4.20"5.40"+1.20"+28.6%
Circumference92.38"99.92"+7.54"+8.2%
Revolutions per mile685.9634.1−51.8−7.6%

Speedometer difference

If your vehicle came with 305/35R21 and you fit 305/45R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.6 mph+1.6 mph
30 mph32.5 mph+2.5 mph
40 mph43.3 mph+3.3 mph
50 mph54.1 mph+4.1 mph
60 mph64.9 mph+4.9 mph
65 mph70.3 mph+5.3 mph
70 mph75.7 mph+5.7 mph
75 mph81.1 mph+6.1 mph
80 mph86.5 mph+6.5 mph

Each size on its own

Questions

Which is bigger, 305/35R21 or 305/45R21?
305/45R21 is taller by 2.40 inches (31.81" vs 29.41"). 305/35R21 is the same width.
Can I replace 305/35R21 with 305/45R21?
It is not recommended without checking carefully. The diameter difference is +8.2%, 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 305/35R21), your real speed is 64.9 mph. Your odometer will under-count miles by about 8.2%.
Will 305/45R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.20 inches of ground clearance at the axle.

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