305/60R18 vs 275/70R18

275/70R18 is 0.7" taller than 305/60R18 (2.3%)

Diameter difference+2.31%diameter 32.41" → 33.16"
305/60R18275/70R18
305/60R18275/70R18Difference
Overall diameter32.41"33.16"+0.75"
Section width12.01"10.83"−1.18"
Sidewall height7.20"7.58"+0.37"
Circumference101.8"104.2"+2.4"
Revolutions per mile622608−14
Wheel diameter18"18"±0"

Speedometer

ShowsActualDiff
30 mph30.7 mph+0.7
45 mph46.0 mph+1.0
60 mph61.4 mph+1.4
70 mph71.6 mph+1.6
80 mph81.8 mph+1.8

Gear ratio to match

CurrentEquivalent
3.423.50
3.553.63
3.733.82
4.104.19

275/70R18 is 0.75" taller than 305/60R18, a +2.31% change in diameter, inside the 3% range most shops treat as a safe swap.

Ground clearance changes by 0.37" (half the diameter change), and the tire needs 0.37" more 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 0.37" taller on 275/70R18.

Details: 305/60R18 in inches · 275/70R18 in inches · change sizes in the calculator

FAQ

What is the difference between 305/60R18 and 275/70R18?

275/70R18 is 0.75" taller (+2.31%) and 1.18" narrower than 305/60R18.

Will 275/70R18 change my speedometer?

Yes, by +2.31%. When the speedometer shows 60 mph you would be going 61.4 mph.

Can I replace 305/60R18 with 275/70R18?

Often yes: the diameter change of +2.3% is within the common 3% guideline. Check the load index, speed rating and clearance.