Well I got brave and decided to try out a more concentrated brew of those corn squeezin's! Pulled up to the pump and put in 6.7 gal of E10. Hung up the hose and got the E85 hose from the other side of the pump and put in 6.7 gal of E85. Car ran good, although maybe a bit weak on the top end, mostly highway, interstate, and a bit of town driving. Topped off the tank with E10 again before going home. Made 18 mpg which is about as good as the car ever does. I'll do it again next chance I get and try to keep a 50/50 ratio of E10/E85 in the tank. This is a '78 Impala 305 2bbl carb. YMMV!