Tire Size Box

225/35R18 vs 205/40R18

205/40R18 compared with 225/35R18

+1.1% diameter, −0.8" width

205/40R18 is 24.46" tall vs 24.20". Within the 3% rule, so usually interchangeable.

24.2"18" rim3.1"8.9" wide
225/35R18 (solid) compared with 205/40R18 (dashed outline), drawn to scale

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

60.6 mph

205/40R18 is +0.26 inches (+1.1%) in overall diameter. That is within the 3% most shops and makers allow.

225/35R18205/40R18Difference%
Overall diameter24.20"24.46"+0.26"+1.1%
Section width8.86"8.07"−0.79"−8.9%
Sidewall height3.10"3.23"+0.13"+4.1%
Circumference76.03"76.83"+0.80"+1.1%
Revolutions per mile833.4824.6−8.7−1.0%

Speedometer difference

If your vehicle came with 225/35R18 and you fit 205/40R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.2 mph+0.2 mph
30 mph30.3 mph+0.3 mph
40 mph40.4 mph+0.4 mph
50 mph50.5 mph+0.5 mph
60 mph60.6 mph+0.6 mph
65 mph65.7 mph+0.7 mph
70 mph70.7 mph+0.7 mph
75 mph75.8 mph+0.8 mph
80 mph80.8 mph+0.8 mph

Each size on its own

Questions

Which is bigger, 225/35R18 or 205/40R18?
205/40R18 is taller by 0.26 inches (24.46" vs 24.20"). 225/35R18 is wider by 0.79 inches.
Can I replace 225/35R18 with 205/40R18?
Usually, yes. The diameter difference is +1.1%, inside the 3% rule of thumb most tire shops use. Check that the wider or taller tire clears your suspension and fenders, and that its load index meets your placard.
How much will 205/40R18 change my speedometer?
When your speedometer reads 60 mph with 205/40R18 fitted (calibrated for 225/35R18), your real speed is 60.6 mph. Your odometer will under-count miles by about 1.1%.
Will 205/40R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.13 inches of ground clearance at the axle.

Other comparisons