Originally Posted by G37Sam

That is not necessarily correct.

To achieve the highest MPG, you need to be running at the most optimum AFR, not too rich nor too lean. It is at the point that your engine is running at it's maximum efficiency. Once you're running at the highest efficiency, you will also be getting the maximum MPG.

How?

Let's say you want to cruise on the highway at 80mph, let's also assume for that to be achieved, you'd have to be doing 2.5k rpm in 6th (gearing designed like that). Let's also assume that you're current AFR (before tune) is at AFR 14:1. Now at 2.5k rpm, the throttle needs to be open at say 30% to let in X amount of air which will combust with X/14 amount of fuel to produce the 100bhp needed to overcome road friction and aerodynamic resistance, just enough bhp to keep that car going at a constant speed (zero acceleration). Now let's say, our engine are most efficient at an AFR of 12.0:1 and you got a tune to get you do 12.0:1 @ 2.5k rpm. In this case, to generate that same 100bhp to do 80mph, you won't need the throttle to be open at 30%, you can now get it to open at 25% only since you're getting a more efficient combustion now. You automatically will be burning less fuel per same distance traveled @ 80mph which is what yields the higher MPG number.

Hope that makes any sense