Why is my car engine temperature low?

The central question is whether the operating temperature is correct or not. If the engine is running too cool it will drink a lot of gas. When it is warmed up, try gripping the upper radiator hose. It should be too hot to wrap your hand around for more than a fraction of a second. A more accurate test can be done with an infrared touch-less thermometer (they are cheap now, but not free) or even by taping a meat thermometer to the upper radiator hose. If the temperature is not at least 180 degrees (F) the new thermostat is not working right. If it is an aftermarket thermostat I am not surprised; I gave up on ever finding a good one many years ago.