My son's "new" 94 Geo Prizm got 37 mpg on its journey to its new home. We drove it 300 miles - half through a driving thunderstorm and half at a cruise-controlled speed of 60 mph, all with a spare tire on the front. He hads since put 4 new tires on it and replaced the air filter. The last two fill-ups showed only 25 mpg. Any ideas on what could have happened or what could be the problem?