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We get asked this a lot, however the answer is simple:

To get more power you need more fuel. You only get this when the accelerator pedal is fully depressed. In any other state, the car would be using only the amount of fuel required to keep at your desired speed. 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 alter the injector calibrations or trip computer, this means your MPG trip computer (if fitted) will read a true value as it did before the remap, any gains you see on this are real.

How to calculate MPG if you have no trip computer?

If you want to calculate your MPG, the best way is to run the car brim to brim (taking note of the fuel quantity required to refill completely) and then run some simple maths: Multiply the litres of diesel by 0.21997 to convert to gallons. You then divide the miles you have covered by the gallons of fuel used.

As an example: Your tank took 47 litres and you covered 550 miles. 47x0.21997 = 10.34 gallons. 550/10.34= 53.2 MPG.