Most owners get around 20 MPG (11.75 L/100km). On-highway use typically results in 25 MPG (9.5 L/100 km). These numbers vary a fair amount according to season, location, altitude, type of driving, state of modifications, age of various parts, and quality of gasoline used; non-turbo owners often have the edge here. Owners have reported 'normal' mileage on both new and used cars that vary from typical by as much as 10 MPG.
Mileage in the 12-13 MPG (18-20 L/100 km) sometimes reflects a problem with the car. An old or failing oxygen sensor is a prime suspect, as this component senses the air/fuel mixture and, when bad (or even weak), can cause the ECU to supply more fuel than is necessary. According to Todd Day of Technomotive, even a slightly weak O2 sensor can knock 1-2 MPG off of the normal mileage. Since weak or dead oxygen sensors often do not trip the "Check Engine" light (see here for why), owners may not realize their O2 sensor is on the blink.
However, this is not always the case. Mileage can often be lower in the winter simply because of the widespread sale of oxygenated gasoline, which tends to lower fuel mileage. Cold weather also leads to increased idling and more congested traffic.
Another common problem occurs when upgraded downpipes are installed. Techs often forget to re-connect the ground strap from the original downpipe, leading to a poor ground for the oxygen sensor that can have the car running rich.
If you're really desparate, try replacing your thermostat - a poorly operating unit can keep the car thinking it's cold so it never leaves it's 'warm-up' mode, during which the ECU runs the car richer than normal.
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