Tire Size Box

325/35R18 vs 325/45R18

325/45R18 compared with 325/35R18

+9.5% diameter, 0.0" width

325/45R18 is 29.52" tall vs 26.96". More than the 3% rule of thumb, so check carefully before switching.

27.0"18" rim4.5"12.8" wide
325/35R18 (solid) compared with 325/45R18 (dashed outline), drawn to scale

With 325/45R18 tires, when your speedometer says 60 mph you are really going

65.7 mph

325/45R18 is +2.56 inches (+9.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/35R18325/45R18Difference%
Overall diameter26.96"29.52"+2.56"+9.5%
Section width12.80"12.80"0.00"0.0%
Sidewall height4.48"5.76"+1.28"+28.6%
Circumference84.69"92.73"+8.04"+9.5%
Revolutions per mile748.2683.3−64.9−8.7%

Speedometer difference

If your vehicle came with 325/35R18 and you fit 325/45R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.9 mph+1.9 mph
30 mph32.8 mph+2.8 mph
40 mph43.8 mph+3.8 mph
50 mph54.7 mph+4.7 mph
60 mph65.7 mph+5.7 mph
65 mph71.2 mph+6.2 mph
70 mph76.6 mph+6.6 mph
75 mph82.1 mph+7.1 mph
80 mph87.6 mph+7.6 mph

Each size on its own

Questions

Which is bigger, 325/35R18 or 325/45R18?
325/45R18 is taller by 2.56 inches (29.52" vs 26.96"). 325/35R18 is the same width.
Can I replace 325/35R18 with 325/45R18?
It is not recommended without checking carefully. The diameter difference is +9.5%, 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 325/45R18 change my speedometer?
When your speedometer reads 60 mph with 325/45R18 fitted (calibrated for 325/35R18), your real speed is 65.7 mph. Your odometer will under-count miles by about 9.5%.
Will 325/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +1.28 inches of ground clearance at the axle.

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