When we remap a vehicle, especially when fuel saving is the priority we focus on improving the low end torque in particular and widening the power-band as far as we can.
This increase in low end torque will mean less throttle pedal input is required to maintain cruising speeds, when fully laden or when on a gradient. It will also mean that the vehicle will be more comfortable in higher gears at slower speeds as well requiring fewer gear changes overall.
If you combine these factors with adjusting your driving style to make the most of the extra low end torque you will see an improvement in fuel consumption. This is most effective on turbo-diesel engines where the savings could be as much as 20%!