Tire Size Box

255/35R18 vs 255/45R18

255/45R18 compared with 255/35R18

+8.0% diameter, 0.0" width

255/45R18 is 27.04" tall vs 25.03". More than the 3% rule of thumb, so check carefully before switching.

25.0"18" rim3.5"10.0" wide
255/35R18 (solid) compared with 255/45R18 (dashed outline), drawn to scale

With 255/45R18 tires, when your speedometer says 60 mph you are really going

64.8 mph

255/45R18 is +2.01 inches (+8.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/35R18255/45R18Difference%
Overall diameter25.03"27.04"+2.01"+8.0%
Section width10.04"10.04"0.00"0.0%
Sidewall height3.51"4.52"+1.00"+28.6%
Circumference78.63"84.93"+6.31"+8.0%
Revolutions per mile805.8746.0−59.8−7.4%

Speedometer difference

If your vehicle came with 255/35R18 and you fit 255/45R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.6 mph+1.6 mph
30 mph32.4 mph+2.4 mph
40 mph43.2 mph+3.2 mph
50 mph54.0 mph+4.0 mph
60 mph64.8 mph+4.8 mph
65 mph70.2 mph+5.2 mph
70 mph75.6 mph+5.6 mph
75 mph81.0 mph+6.0 mph
80 mph86.4 mph+6.4 mph

Each size on its own

Questions

Which is bigger, 255/35R18 or 255/45R18?
255/45R18 is taller by 2.01 inches (27.04" vs 25.03"). 255/35R18 is the same width.
Can I replace 255/35R18 with 255/45R18?
It is not recommended without checking carefully. The diameter difference is +8.0%, 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 255/45R18 change my speedometer?
When your speedometer reads 60 mph with 255/45R18 fitted (calibrated for 255/35R18), your real speed is 64.8 mph. Your odometer will under-count miles by about 8.0%.
Will 255/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +1.00 inches of ground clearance at the axle.

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