Tire Size Box

305/35R18 vs 305/45R18

305/45R18 compared with 305/35R18

+9.1% diameter, 0.0" width

305/45R18 is 28.81" tall vs 26.41". More than the 3% rule of thumb, so check carefully before switching.

26.4"18" rim4.2"12.0" wide
305/35R18 (solid) compared with 305/45R18 (dashed outline), drawn to scale

With 305/45R18 tires, when your speedometer says 60 mph you are really going

65.5 mph

305/45R18 is +2.40 inches (+9.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/35R18305/45R18Difference%
Overall diameter26.41"28.81"+2.40"+9.1%
Section width12.01"12.01"0.00"0.0%
Sidewall height4.20"5.40"+1.20"+28.6%
Circumference82.96"90.50"+7.54"+9.1%
Revolutions per mile763.8700.1−63.7−8.3%

Speedometer difference

If your vehicle came with 305/35R18 and you fit 305/45R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.8 mph+1.8 mph
30 mph32.7 mph+2.7 mph
40 mph43.6 mph+3.6 mph
50 mph54.5 mph+4.5 mph
60 mph65.5 mph+5.5 mph
65 mph70.9 mph+5.9 mph
70 mph76.4 mph+6.4 mph
75 mph81.8 mph+6.8 mph
80 mph87.3 mph+7.3 mph

Each size on its own

Questions

Which is bigger, 305/35R18 or 305/45R18?
305/45R18 is taller by 2.40 inches (28.81" vs 26.41"). 305/35R18 is the same width.
Can I replace 305/35R18 with 305/45R18?
It is not recommended without checking carefully. The diameter difference is +9.1%, 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 305/45R18 change my speedometer?
When your speedometer reads 60 mph with 305/45R18 fitted (calibrated for 305/35R18), your real speed is 65.5 mph. Your odometer will under-count miles by about 9.1%.
Will 305/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +1.20 inches of ground clearance at the axle.

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