Correct - eliminate the obvious and then get a baseline..did the mileage drop suddenly or over a time period ? Have you cleaned the induction system ? Have you taken "live" ODBC readings with a product like the Blue Driver scanner to look for anomalies in fuel distribution/use ?How about slowing down and testing a few other things first?? Valve adjustment? Brakes dragging? (calipers float on pins that are notorious for seizing up and may cause dragging). Tire condition and inflation? Are there any codes??