To keep the odometer/speedo as accurate as possible and gearing as close to stock. The odometer is very accurate w/ the stock sized 275/60R18's. On a recent run, my GPS measured 529 mi, odometer measured 534 mi.
I don't know Jim this 1.6% larger size would make my warranty last just that much longer

Tire Size Comparison
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
265/65-18 6.8in 15.8in 31.6in 99.2in 639 0.0%
275/65-18 7.0in 16.0in 32.1in 100.8in 629 1.6%