You might not think that running your vehicle without a gas cap will cause a drop in fuel economy, but it can. You know gasoline as a liquid, but in truth it does its work best as a vapor. In fact, gasoline or petrol evaporates very quickly. If you don’t believe me, set out a small container of it and leave it there for a day. Come back the next day and it’s probably gone. The same thing can happen inside your fuel tank. The evaporative emission system on your vehicle is designed to help eliminate fuel evaporation into the atmosphere. If this system has an issue, it can effect your fuel mileage. Even though your vehicle seems to run fine with that check engine light for an EVAP problem, it could still be costing you fuel economy. So if you have poor MPG and a check engine light for an evap problem, you should address it sooner rather than later. More information about finding and fixing leaks with the evap system can be found in the article on Finding and Fixing Leaks on Your Vehicle.