Q: How full should I keep my gas tank?

In my opinion, you should keep your tank at least half full at all times. This is not for any mechanical reasons but more for personal safety reasons. If you treat the halfway mark as if that were the empty mark, then when an unforeseen event happens you will have a cushion of fuel. Emergencies happen without warning and the security this provides will bring peace of mind. In foul weather you could become stranded on the highway and run out of fuel. In extreme weather this can pose a threat to your health. A fuel or gas gauge is not exact and cannot guarantee how many miles are actually left until the tank is empty. Some people think that there are still many more miles left once the gauge is at E, while others swear that once the Low Level Light is on, if they don’t get fuel they may run out before they get to the next station. A vehicle breaking down cannot usually be avoided since parts can break or fail without warning, but you can avoid damage or the inconvenience of running out of fuel if you simply keep the tank filled.

From a mechanical point of view, modern vehicles have evaporative emissions systems to prevent fuel vapor leaks from the tank. In order to test these systems, criteria need to be met before the test can be performed. Many of these systems have temperature and level requirements in order to run the self diagnostic check of the system. You may have heard someone complain that the Check Engine Light came on in their car after getting gas due to a loose cap. This emissions monitor system generally requires the tank to be anywhere between 15 to 80 percent full in order to run the self diagnosis test properly. If the tank is constantly topped off or left empty, these tests may fail to run. For your convenience, safety and for the onboard monitor systems, keep your tank full.