In my gassers I lose 2-3 mpg. Used to lose more with carbureted engines.
Today it's a bit different in that fuel injected engines don't flood, in general (anybody remember sticking screwdrivers down carb choke plates to hold em open?), so don't waste as much uncombusted fuel. But some of that has been offset by EPA-mandated low RVP summer gas, which means summer gas is denser and yields more mpg than winter gas. Further, we didn't have farm lobby-mandated alcohol gas before, which has low BTU energy density. Gasoline has become a real witch's brew today.
Diesel Fuel has it's own political and pollution issues. Stripped of sulfur and heavy ends in 1994, sulfur replacement lubricity additives, farm lobby-mandated soybean oil added, biocides for biodiesel storage, all contributed to much higher cost and lower energy density. At least it burns cleaner, I guess.