1998 Pontiac Transport gas mileage suddenly dropped but gar

I used to get about 700 - 750 km to a tank of gas when using regular grade. I switched to premium a few years ago and have generally gotten around 900 km, sometmes slightly higher, to a tank especially if mostly highway driving. Three tankfuls ago I only barely got to 700 km on a tank. The next tank about the same. I had my garage look at it, run tests etc. They couldn't find any problems with their computer or visual checks but changed the oil, air and gas filters just in case. Tire pressures were right on. This tankful is worse than before as I will likely not make anywhere near 700 km on it, always with premium gas.

What could cause my van to suddenly give such poor mileage and yet run perfectly and show nothing wrong on computer or visual checks? What further tests can be run to isolate the problem? Thank you for your time. I have another question. Not sure if you will answer it here or require me to return to this page.

I have a problem with one of the wheels or tires on my van. It is causing a strange up and down type of vibration. I have had two reasonably new tires exchanged for two brand new ones. The problem persists. People have checked the aliginment, wheel balance, wheels out of round, tire separation etc but not been able to find it. One test drive resulted in someone telling me that my front rotors needed to be machined. Another mechanic said it couldn't be that because there's no vibration while braking. Any suggestions? Again, thank you in advance.