I have a 1996 Corvette that I have driven for 11 years. I have always gotten 18-20 highway mpg. In the last few weeks, that has dropped to only about 15 mpg. My tires are correctly inflated and the air/fuel filters are new. I had a tune up with new spark plugs, etc. agout three months ago. The distributer was recently replaced, but this was after the gas mileage dropped. I do not think gas is being stolen from the tank. Any other ideas or advice? Thanks.
If every other mechanical possibility has been ruled out, then the thing to check is the thermostat. If it is stuck in the open position, the engine may be chronically running at a colder temperature than it was designed for, and this will lead to running on a richer gas/air mixture–hence, lower mpg.
When O2 sensors start to go bad, they can reduce gas mileage and may not throw a check engine light until they fail completely.
It might be possible that local gas stations have begun using ethanol in their fuel, and that could explain the mileage reduction, because ethanol has less energy than gasoline, so even when mixed to a 10% concentration in gas, will result in reduced mileage.