Comment by Hamuko

You won’t get better fuel efficiency unless your manual gearbox has eight (or even nine or ten) gears like the automatics have now. For example, BMW says that the manual BMW M2 does 10.0–10.2 l/100km and the eight-speed automatic does 9.6–9.8 l/100 km. It’s also slightly slower and has higher emissions (since it burns more fuel too).