A nice side bonus of tuning your car for more power is that in many circumstances you'll also save a little fuel. Of course, this is all within reason and depends entirely on your driving style after remapping.
If you drive the vehicle harder constantly you will use more fuel, it's where the power comes from. Once you've gotten used to your new found power and you learn to keep the RPMs down and drive on the torque you'll hopefully start to see some small fuel economy improvements.
We get asked this a lot, however the answer is actually quite simple. To get more power you do need more fuel but you only use more power when the accelerator pedal is depressed. In any other state, the car would be using only the amount of fuel required to keep at your desired speed and the fuel economy can be improved through a remap.
With a small advance to the start of the injection period it is possible to get more power from the same amount of fuel thanks to increased peak cylinder pressures. This means that you can get the same amount of power from less fuel when compared to before. Due to this, it is possible to use less fuel on your normal journeys as your engine will be running more efficiently.
The fuel pressure can also be increased very slightly throughout the rev range to promote better atomisation of the fuel. This will increase the surface area to volume ratio of the fuel droplets, helping it to vaporise and burn under more complete stoichiometric conditions. This effectively means the same as a timing advance. More power from the same amount of fuel, hence reduced fuel consumption.
Finally a large proportion of the fuel savings will come from increased engine torque at lower engine speeds; the result of this is you can usually drive one gear higher thanks to the extra torque, this reduces engine speed and saves fuel.
At HDI Tuning Ltd we NEVER rig the trip computer, this means your MPG or KM trip computer (if fitted) will read a true value as it did before the remap, any gains you see on this are real.