I have a 2007 Toyota Rav4, ~45k miles. Recently, since mid-October my mileagge has droped from a consistent 25MPG to 22-23MPG. Could a change in the gas mixture for winter cause this reduction? I have change my air filter regularly, but have had a tune up or other maintenance performed.
It also could be the cold mornings we’ve had since mid-October. A cold engine uses more fuel until it warms up. The colder it is, the more fuel it uses.
In addition to other weather-related causes, there is also the likelihood that your tires need air. Tires lose 1 lb of pressure for every 10 degree drop in temperatures, meaning that they could well be 3 or 4 lbs underinflated by this time.
The engine has a component called a Mass Air Flow sensor. This measures the volume or the mass of the air entering the engine. The computer then determines how much fuel to inject into the engine from this information. Since cold air is denser or has more mass this sensor detects this so the computer injects more fuel, and the fuel mileage drops slightly.