There is no standard for which side a gas cap must be on, so engineers place them where it's most convenient for the design. In cars with a single exhaust pipe, the cap will (almost) always be on the side opposite of the pipe to avoid the chance of igniting spilled gas. Newer cars will often have a small arrow next to the gas pump logo on the fuel gauge, telling you which side of the car the gas cap is on.
Many older cars have the gas tank in the rear of the car, with the gas cap in the center above the bumper. While this was convenient, it also meant that a severe rear-end crash could result in a ruptured tank. The Ford Pinto was noted for this, as a lack of any safety structure behind the tank meant a rear end collision over 25mph would result in an explosion:
1973-1987 Chevy trucks had a similar problem: they had side-mounted tanks that would rupture in a t-bone accident.
Today, car gas tanks are placed underneath the rear seat, while truck tanks are between the frame rails. This makes it impossible to mount a center gas cap.
If this really irks you, look for a pre-1988 Jaguar XJ. These cars have a gas cap on each side, so no matter how you pull up to the pump, the hose will always reach.
Posted 1347 day ago