Tire Size Box

205/35R18 vs 205/40R18

205/40R18 compared with 205/35R18

+3.4% diameter, 0.0" width

205/40R18 is 24.46" tall vs 23.65". More than the 3% rule of thumb, so check carefully before switching.

23.6"18" rim2.8"8.1" wide
205/35R18 (solid) compared with 205/40R18 (dashed outline), drawn to scale

With 205/40R18 tires, when your speedometer says 60 mph you are really going

62.0 mph

205/40R18 is +0.81 inches (+3.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

205/35R18205/40R18Difference%
Overall diameter23.65"24.46"+0.81"+3.4%
Section width8.07"8.07"0.00"0.0%
Sidewall height2.82"3.23"+0.40"+14.3%
Circumference74.30"76.83"+2.54"+3.4%
Revolutions per mile852.8824.6−28.1−3.3%

Speedometer difference

If your vehicle came with 205/35R18 and you fit 205/40R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.0 mph+1.0 mph
40 mph41.4 mph+1.4 mph
50 mph51.7 mph+1.7 mph
60 mph62.0 mph+2.0 mph
65 mph67.2 mph+2.2 mph
70 mph72.4 mph+2.4 mph
75 mph77.6 mph+2.6 mph
80 mph82.7 mph+2.7 mph

Each size on its own

Questions

Which is bigger, 205/35R18 or 205/40R18?
205/40R18 is taller by 0.81 inches (24.46" vs 23.65"). 205/35R18 is the same width.
Can I replace 205/35R18 with 205/40R18?
It is not recommended without checking carefully. The diameter difference is +3.4%, 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 205/40R18 change my speedometer?
When your speedometer reads 60 mph with 205/40R18 fitted (calibrated for 205/35R18), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.4%.
Will 205/40R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.40 inches of ground clearance at the axle.

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