Sudden reduction in MPG

[schuh8]

Hi, I'm new to the forum and have a fuel economy issue I really need help with.

Car in question is a 2002 Chevy Venture van, with 200,000 miles on the od. I have kept careful records in the 90,000 miles I have owned the car and it has averaged 21 to 22 mpg (combined city/highway) and 25 to 27 mpg highway.Better in winter than summer of course.

Recently (and rather suddenly), both in town and highway mileage has dropped about 3 mpg.There is no question about this as I have an ultra gage and a OEM in car mpg computer and both are backed up by manual calculations.

New plugs, new oxygen sensor, new air filter, new gas filter, same gas station, same air pressure in the tires (40 psi) ..... and nothing has made a bit of difference. It has been very warm here (mid 90's) but that happens every year (just a little later).

Can anybody tell me where to look next ?