[Ecky]

I'll first state that, on an individual level, I'm not overly concerned with emissions. Don't take this as me advocating for not tuning from a pollution standpoint - I'm not.

I'm skeptical that pulling 10% fuel, which would also result in an approximate 10% reduction in torque/power and require more throttle under the same conditions, would result in a 20% improvement in fuel economy.

The G1 Insight had a second catalytic converter with a NOx trap, specifically to capture the pollutants created by lean burn. When cruising on the highway, it would drop out of lean burn for maybe ~5 seconds every 5 minutes, to purge the second catalyst. When it dropped out of lean burn, you'd see instant fuel economy drop from 100mpg to ~75mpg. However, you'd also start to gain speed, so you would need to pull the throttle back. Ultimately, at the same speed, non-lean cruise would get 90-95mpg where the lean cruise mode was 95-100mpg.

Similarly, I spent around a year fine tuning a lean tune on a different engine. Along with timing and VVT adjustments, I was able to see perhaps 1-2mpg in a car that was otherwise getting around 45mpg.

I'm not saying a 20% improvement is impossible from lean tuning alone (though the maths suggest it nearly is). Just, often people make a change, and then they also unconsciously change their habits, and together these add up to a larger improvement.

EDIT: As I understand it, most of the gains from lean tuning come from having a larger volume of "inert" gas to heat and expand in the cylinder, and also to create less power at a given throttle opening, so you open the throttle more, which reduces vacuum and related losses. These are not insignificant, but still relatively marginal gains.