Tire Size Box

275/35R18 vs 275/45R18

275/45R18 compared with 275/35R18

+8.5% diameter, 0.0" width

275/45R18 is 27.74" tall vs 25.58". More than the 3% rule of thumb, so check carefully before switching.

25.6"18" rim3.8"10.8" wide
275/35R18 (solid) compared with 275/45R18 (dashed outline), drawn to scale

With 275/45R18 tires, when your speedometer says 60 mph you are really going

65.1 mph

275/45R18 is +2.17 inches (+8.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

275/35R18275/45R18Difference%
Overall diameter25.58"27.74"+2.17"+8.5%
Section width10.83"10.83"0.00"0.0%
Sidewall height3.79"4.87"+1.08"+28.6%
Circumference80.36"87.16"+6.80"+8.5%
Revolutions per mile788.5726.9−61.5−7.8%

Speedometer difference

If your vehicle came with 275/35R18 and you fit 275/45R18, 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, 275/35R18 or 275/45R18?
275/45R18 is taller by 2.17 inches (27.74" vs 25.58"). 275/35R18 is the same width.
Can I replace 275/35R18 with 275/45R18?
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 275/45R18 change my speedometer?
When your speedometer reads 60 mph with 275/45R18 fitted (calibrated for 275/35R18), your real speed is 65.1 mph. Your odometer will under-count miles by about 8.5%.
Will 275/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +1.08 inches of ground clearance at the axle.

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