This is a question that comes across just about everyone’s mind who is considering making their car faster, and the answer is…. Well, it depends.
Generally speaking, when you are putting on various bolt on parts to increase your engine’s power out-put, the main way that you are accomplishing this goal is by allowing the engine to operate more efficiently.
A perfect example of this would be an intake, or an exhaust.
What you are doing in this case is allowing the engine to pull in or push out, air with less resistance. This means that more of the power generated by the engine goes to moving you forward.
Another example of this is an ECU Re-Flash. Cars that come with forced induction from the factory have a tendency to run very rich. This is because it increases the margin of safety for your motor, but often times, car manufacturers go a little too far. One thing that a Re-Flash often does is to lean out the fuel a little bit to make more power. By doing this, you are injecting a little less fuel all the time, and this also has the potential to increase fuel economy.
Because of this, putting these parts on will not only increase the amount of power that your engine produces, but it can also increase your fuel economy. But that isn’t always the case.
People don’t typically put these parts on with the goal of increasing fuel economy, they usually want to go faster. If you are going to drive aggressively, your fuel economy will be bad no matter what you do, and this is especially true in a car with forced induction.
If your car has a turbo, the more that you in positive boost, the more air that you are injecting into your motor, which also means that you are injecting more fuel. So the more that you mash the “Go” pedal, the worse your mileage will be.
We experienced all of this in our Project Car, which is a 2003 Subaru WRX. Before we did a thing to it, we were getting about 24-25 mpg. After going up to a Cobb Stage 2 (engine management and a Turbo-Back Exhaust), we actually saw our mileage go up to about 27 mpg on average, and to almost 29 mpg on a long drive. At the track however (Which is mostly wide open throttle), we have seen as bad as 12 mpg…
So the moral of the story is that you are actually in control of your own destiny when it comes to mileage, but if you take it easy, you might be able to have a fast car, and drive it too.