Simply put, the Avalanche, using E85, got three miles per gallon WORSE fuel mileage. We clocked an average fuel economy of 13.4 miles per gallon using regular grade unleaded – not so hot, but much better than E85 that registered just 10.4 miles per gallon under virtually identical driving conditions. This shorter range and virtually identical price per gallon makes the use of E85 more expensive by around $20 more per tank.