275/65R18 vs 275/70R18

Nominal diameter rises from 32.1 in (815 mm) to 33.2 in (842 mm). Section width stays at 10.8 in (275 mm), while sidewall height rises from 7.0 in (179 mm) to 7.6 in (193 mm).

By the formula, revolutions per mile fall from 629 to 608. Ground clearance at the axle increases by 0.54 in (14 mm). These are unloaded nominal dimensions; actual tires measure differently.

With calibration unchanged, a nominal change to 275/70R18 means 60 mph indicated becomes 62.0 mph actual. At 100 km/h indicated, actual speed is 103.4 km/h. Actual distance traveled is also 3.4% greater than the odometer records.

By the formula, at 65 mph with a 3.73 axle ratio and a 1.00 transmission gear, engine speed falls from 2,541 rpm to 2,458 rpm, down 3.3%. Restoring the original engine speed mathematically requires a 3.86 axle ratio: 3.73 × 33.16 / 32.07. This is a calculation, not a modification recommendation.

The Firestone, Nitto, Kumho, Toyo and Mickey Thompson size tables list 28 tires in 275/65R18: 16 LT and 12 P-metric or passenger versions. Published diameters span 32.09–32.4 in, with SL, XL and E load ranges listed.

Those makers list 20 tires in 275/70R18: 19 LT and 1 P-metric or passenger version. Published diameters span 33.19–33.62 in, with SL and E load ranges listed. For the Open Country A/T III, Toyo lists 647 revolutions per mile in LT275/65R18 and 626 in LT275/70R18, distinct from the nominal calculations.

For more nominal axle clearance and sidewall height, 275/70R18 adds both, along with lower engine rpm at the same road speed. Keeping 275/65R18 retains the shorter diameter and its speedometer and gearing relationship.

This is a taller-versus-shorter comparison, not a wider-versus-narrower change by the formula. Actual fit depends on the vehicle, wheels and suspension. Compare the specific maker's dimensions and approved rim widths, not just the size label.