It hardly seems worth mentioning, but I will anyway. If you have any fuel leaks in your vehicle, it will affect your MPG. If the fuel never makes it to the engine, it can’t be burned. If you have a fuel leak, address it sooner rather than later — not just because it’s affecting your gas mileage, but because it’s a safety issue as well. More information on finding and fixing leaks with the evaporative emissions system can be found in the article on Finding and Fixing Leaks on Your Vehicle.