255/35R18 vs 255/40R18

255/40R18 is 1.0" taller than 255/35R18 (4.0%)

Diameter difference+4.01%diameter 25.03" → 26.03"
255/35R18255/40R18
255/35R18255/40R18Difference
Overall diameter25.03"26.03"+1.00"
Section width10.04"10.04"±0.00"
Sidewall height3.51"4.02"+0.50"
Circumference78.6"81.8"+3.2"
Revolutions per mile806775−31
Wheel diameter18"18"±0"

Speedometer

ShowsActualDiff
30 mph31.2 mph+1.2
45 mph46.8 mph+1.8
60 mph62.4 mph+2.4
70 mph72.8 mph+2.8
80 mph83.2 mph+3.2

Gear ratio to match

CurrentEquivalent
3.423.56
3.553.69
3.733.88
4.104.26

255/40R18 is 1.00" taller than 255/35R18, a +4.01% change in diameter. That is more than the 3% many tire shops use as a limit, so plan on recalibrating the speedometer and checking ABS and stability control.

Ground clearance changes by 0.50" (half the diameter change), and the tire needs 0.50" more room above it.

Both tires are the same width, 10.04".

Both fit a 18" wheel; the sidewall is 0.50" taller on 255/40R18.

Details: 255/35R18 in inches · 255/40R18 in inches · change sizes in the calculator

FAQ

What is the difference between 255/35R18 and 255/40R18?

255/40R18 is 1.00" taller (+4.01%) and 0.00" wider than 255/35R18.

Will 255/40R18 change my speedometer?

Yes, by +4.01%. When the speedometer shows 60 mph you would be going 62.4 mph.

Can I replace 255/35R18 with 255/40R18?

It is a +4.0% diameter change, larger than the common 3% guideline, so check clearance and plan to recalibrate.