325/65R18 vs 275/70R18

325/65R18 is 1.5" taller than 275/70R18 (4.3%)

Diameter difference−4.26%diameter 34.63" → 33.16"
325/65R18275/70R18
325/65R18275/70R18Difference
Overall diameter34.63"33.16"−1.48"
Section width12.80"10.83"−1.97"
Sidewall height8.32"7.58"−0.74"
Circumference108.8"104.2"−4.6"
Revolutions per mile582608+26
Wheel diameter18"18"±0"

Speedometer

ShowsActualDiff
30 mph28.7 mph−1.3
45 mph43.1 mph−1.9
60 mph57.4 mph−2.6
70 mph67.0 mph−3.0
80 mph76.6 mph−3.4

Gear ratio to match

CurrentEquivalent
3.423.27
3.553.40
3.733.57
4.103.93

275/70R18 is 1.48" shorter than 325/65R18, a −4.26% 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 0.74" (half the diameter change), and the tire needs 0.74" less room above it.

325/65R18 is 1.97" wider. On the same wheel offset about half of that, 0.98", moves toward the suspension and half toward the fender.

Both fit a 18" wheel; the sidewall is 0.74" shorter on 275/70R18.

Details: 325/65R18 in inches · 275/70R18 in inches · change sizes in the calculator

FAQ

What is the difference between 325/65R18 and 275/70R18?

275/70R18 is 1.48" shorter (−4.26%) and 1.97" narrower than 325/65R18.

Will 275/70R18 change my speedometer?

Yes, by −4.26%. When the speedometer shows 60 mph you would be going 57.4 mph.

Can I replace 325/65R18 with 275/70R18?

It is a −4.3% diameter change, larger than the common 3% guideline, so check clearance and plan to recalibrate.