Tire Size Box

235/35R18 vs 235/45R18

235/45R18 compared with 235/35R18

+7.6% diameter, 0.0" width

235/45R18 is 26.33" tall vs 24.48". More than the 3% rule of thumb, so check carefully before switching.

24.5"18" rim3.2"9.3" wide
235/35R18 (solid) compared with 235/45R18 (dashed outline), drawn to scale

With 235/45R18 tires, when your speedometer says 60 mph you are really going

64.5 mph

235/45R18 is +1.85 inches (+7.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/35R18235/45R18Difference%
Overall diameter24.48"26.33"+1.85"+7.6%
Section width9.25"9.25"0.00"0.0%
Sidewall height3.24"4.16"+0.93"+28.6%
Circumference76.89"82.71"+5.81"+7.6%
Revolutions per mile824.0766.1−57.9−7.0%

Speedometer difference

If your vehicle came with 235/35R18 and you fit 235/45R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.5 mph+1.5 mph
30 mph32.3 mph+2.3 mph
40 mph43.0 mph+3.0 mph
50 mph53.8 mph+3.8 mph
60 mph64.5 mph+4.5 mph
65 mph69.9 mph+4.9 mph
70 mph75.3 mph+5.3 mph
75 mph80.7 mph+5.7 mph
80 mph86.0 mph+6.0 mph

Each size on its own

Questions

Which is bigger, 235/35R18 or 235/45R18?
235/45R18 is taller by 1.85 inches (26.33" vs 24.48"). 235/35R18 is the same width.
Can I replace 235/35R18 with 235/45R18?
It is not recommended without checking carefully. The diameter difference is +7.6%, 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 235/45R18 change my speedometer?
When your speedometer reads 60 mph with 235/45R18 fitted (calibrated for 235/35R18), your real speed is 64.5 mph. Your odometer will under-count miles by about 7.6%.
Will 235/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.93 inches of ground clearance at the axle.

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