What is amazing is this guy can get away with saying this crap. I've owned Turbo Charged cars since 1981 and know what you can and can't do with them to improve gas mileage.
Slapping on a Turbo with out making changes to the fuel system is a recipe for disaster. Boost means an increase in cylinder and piston temperatures.
You have two choices to cool the cylinder add more fuel or add a water/alcohol injection to cool it. Either way you are not going to get better fuel mileage. Adding fuel mean less gas mileage and adding a water/alcohol mixture means less power.
Sure adding a turbo will increase fuel mileage if you drop the cubic inches of the engine. I can't tell you the people I have told not to just slap a low pressure turbo (4-6 psi) on a stock engine. I'd say 75% had major repair bills with a year or two or the car just never ran well from that point on. One the repair bills usually involved a new head gasket or a hole through a piston due to running a lean mixture. They never increased the fuel mixture under boost to compensate for the increased heat and pressure involved with adding the turbo charger. I've never ever seen someone get better gas mileage by adding a turbocharger to any car.
I could get 40 mpg in a Corvette with a turbo charged engine. I would pull the
5.7 liter and drop in a Buick 3.8L V6 Grand National engine.
