Originally Posted by Chip

I know several people who have done a MPG test on evolution chips, and all have had improvements, to be honest if you sit at the same speed down the motorway (ie cruise control on 80mph) its pretty much impossible for it not to give gains due to what the remap actually does.
As its now working the fuel a lot harder than before (thats where the power comes from) it means it needs less fuel to make the same power, and your car still needs the same power to sit at 80mph that it did before, so if it can make that power on less fuel now, you will lose less fuel by definition.

Some cheaper remaps dont alter the diesel injector timing which is what is critical to increasing efficiency, they just up the boost instead (ie more like how you would tune a petrol turbo), and on those you'll see no difference when sat on cruise control as nothing has changed, and when booting it around you'll get worse economy than before as obviously its putting in more fuel with the extra boost. But it will still make more power, so to drive you wouldnt really know the difference between the two methods.