Interesting- are you just running a stage 1 remap? Did the mpg drop? I think that atmospheric conditions play a huge part in mpg. On the same run to work, same driving style etc etc, I can see ave mpg range from 39 -43…the only difference is the weather - or more specifically, the temperature and/ or moisture content in the air….
all I know, is that harrier jumpjets had a system to rapidly boost power when needed for the ‘hover’, which was to squirt water into the engine. presumably, the instantaneous vaporisation of the water to gas boosted the volume of the air out of the jet for the same amount of fuel…
anyway, I digress..