Cruise performance gone???

My latest flight in the Cessna 182:Weather: 74 degrees at sea-level, and I noted 53 degrees F at my cruise altitude of 6,500 feet. This converts to about 12 degrees Celsius.Barometer was 29.98.Weights:I was flying with a "passenger," so the load was two 190 lb passengers in the front, all else empty. 75% fuel.Speeds:All cruising at 6,500 feet and wide open throttle, giving me 23" manifold pressure. Leaned appropriately.121 KIAS, but that meant a true airspeed of 135 KTAS.GPS groundspeed had me at 141 knots because of winds aloft.This is all pretty accurate.Dan