Tire Size Box

215/40R17 vs 205/35R18

205/35R18 compared with 215/40R17

−0.5% diameter, −0.4" width

205/35R18 is 23.65" tall vs 23.77". Within the 3% rule, so usually interchangeable.

23.8"17" rim3.4"8.5" wide
215/40R17 (solid) compared with 205/35R18 (dashed outline), drawn to scale

With 205/35R18 tires, when your speedometer says 60 mph you are really going

59.7 mph

205/35R18 is −0.12 inches (−0.5%) in overall diameter. That is within the 3% most shops and makers allow.

215/40R17205/35R18Difference%
Overall diameter23.77"23.65"−0.12"−0.5%
Section width8.46"8.07"−0.39"−4.7%
Sidewall height3.39"2.82"−0.56"−16.6%
Circumference74.68"74.30"−0.38"−0.5%
Revolutions per mile848.4852.8+4.4+0.5%

Speedometer difference

If your vehicle came with 215/40R17 and you fit 205/35R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph19.9 mph−0.1 mph
30 mph29.8 mph−0.2 mph
40 mph39.8 mph−0.2 mph
50 mph49.7 mph−0.3 mph
60 mph59.7 mph−0.3 mph
65 mph64.7 mph−0.3 mph
70 mph69.6 mph−0.4 mph
75 mph74.6 mph−0.4 mph
80 mph79.6 mph−0.4 mph

Each size on its own

Questions

Which is bigger, 215/40R17 or 205/35R18?
215/40R17 is taller by 0.12 inches (23.77" vs 23.65"). 215/40R17 is wider by 0.39 inches.
Can I replace 215/40R17 with 205/35R18?
Usually, yes. The diameter difference is −0.5%, 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 205/35R18 change my speedometer?
When your speedometer reads 60 mph with 205/35R18 fitted (calibrated for 215/40R17), your real speed is 59.7 mph. Your odometer will over-count miles by about 0.5%.
Will 205/35R18 lift my vehicle?
Ride height changes by half the diameter difference: −0.06 inches of ground clearance at the axle.

Other comparisons