Tire Size Box

235/35R18 vs 235/40R18

235/40R18 compared with 235/35R18

+3.8% diameter, 0.0" width

235/40R18 is 25.40" 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 235/40R18 (dashed outline), drawn to scale

With 235/40R18 tires, when your speedometer says 60 mph you are really going

62.3 mph

235/40R18 is +0.93 inches (+3.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/35R18235/40R18Difference%
Overall diameter24.48"25.40"+0.93"+3.8%
Section width9.25"9.25"0.00"0.0%
Sidewall height3.24"3.70"+0.46"+14.3%
Circumference76.89"79.80"+2.91"+3.8%
Revolutions per mile824.0794.0−30.0−3.6%

Speedometer difference

If your vehicle came with 235/35R18 and you fit 235/40R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.1 mph+1.1 mph
40 mph41.5 mph+1.5 mph
50 mph51.9 mph+1.9 mph
60 mph62.3 mph+2.3 mph
65 mph67.5 mph+2.5 mph
70 mph72.6 mph+2.6 mph
75 mph77.8 mph+2.8 mph
80 mph83.0 mph+3.0 mph

Each size on its own

Questions

Which is bigger, 235/35R18 or 235/40R18?
235/40R18 is taller by 0.93 inches (25.40" vs 24.48"). 235/35R18 is the same width.
Can I replace 235/35R18 with 235/40R18?
It is not recommended without checking carefully. The diameter difference is +3.8%, 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 235/40R18 change my speedometer?
When your speedometer reads 60 mph with 235/40R18 fitted (calibrated for 235/35R18), your real speed is 62.3 mph. Your odometer will under-count miles by about 3.8%.
Will 235/40R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.46 inches of ground clearance at the axle.

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