Do what you want, it's your car! E85 was designed for use in Flex Fuel Vehicles (FFVs) only! Most cars are built to run on fuel enriched with only 10% ethanol and are warranted for its use. Big difference in E10 & E85! More and more vehicles are being made to run on E85 — (a blend containing 85% ethanol, 15% gasoline). To find out which fuel is best for your car, check your owner's manual, or ask your dealer, but I will guarantee you, neither one will recommend E85 for use in your 95 Riviera!
There are many variables that affect fuel economy (weather, car condition, road grade, tire pressure, air conditioner use, etc.), studies indicate that fuel economy with ethanol-enriched fuels may decrease, but not as likely with E10. E85 has approximately 27% less energy then gasoline; so therefore fuel economy loss varies between 10 to 25%. Like I said; do what you want!