305/60R18 vs 275/65R18

305/60R18 is 0.3" taller than 275/65R18 (1.0%)

Diameter difference−1.03%diameter 32.41" → 32.07"
305/60R18275/65R18
305/60R18275/65R18Difference
Overall diameter32.41"32.07"−0.33"
Section width12.01"10.83"−1.18"
Sidewall height7.20"7.04"−0.17"
Circumference101.8"100.8"−1.1"
Revolutions per mile622629+6
Wheel diameter18"18"±0"

Speedometer

ShowsActualDiff
30 mph29.7 mph−0.3
45 mph44.5 mph−0.5
60 mph59.4 mph−0.6
70 mph69.3 mph−0.7
80 mph79.2 mph−0.8

Gear ratio to match

CurrentEquivalent
3.423.38
3.553.51
3.733.69
4.104.06

275/65R18 is 0.33" shorter than 305/60R18, a −1.03% change in diameter, inside the 3% range most shops treat as a safe swap.

Ground clearance changes by 0.17" (half the diameter change), and the tire needs 0.17" less room above it.

305/60R18 is 1.18" wider. On the same wheel offset about half of that, 0.59", moves toward the suspension and half toward the fender.

Both fit a 18" wheel; the sidewall is 0.17" shorter on 275/65R18.

Details: 305/60R18 in inches · 275/65R18 in inches · change sizes in the calculator

FAQ

What is the difference between 305/60R18 and 275/65R18?

275/65R18 is 0.33" shorter (−1.03%) and 1.18" narrower than 305/60R18.

Will 275/65R18 change my speedometer?

Yes, by −1.03%. When the speedometer shows 60 mph you would be going 59.4 mph.

Can I replace 305/60R18 with 275/65R18?

Often yes: the diameter change of −1.0% is within the common 3% guideline. Check the load index, speed rating and clearance.