again, i understood that part. i was responding to holdens implication that the fooling of the o2 sensor was what was causing the improvements in mileage.
The following data is from
http://www.knfilters.com/airfuelmonitors.htm ----------- ECONOMY BEST ALL-AROUND POWER
Light 1 -Light 2 Light 3 Light 4 Light 5 Light 6 Light 7 Light 8 Light 9 Light 10
Gasoline 17.1 16.0 15.1 14.7 14.7 14.7 14.7 14.0 13.2 12.1
Alcohol 7.6 7.1 6.7 6.5 6.5 6.5 6.5 6.1 5.8 5.3
Propane 17.9 16.8 15.9 15.6 15.6 15.6 15.6 15.0 14.0 13.0
A leaner mixture of 17.1 'economy ratio' vs a 14.7 'best all around' ratio would mean that each displacement of the engine would pull in 1/17.1 vs 1/14.7 or an 16% fuel savings.
If your car was running in the fuel full rich power zone, 1/17 to 1/12 yeilds a 41% fuel burn savings.
41% -- 45% spooky.
Of course pulling a trailer up hill or 0-60 time would be affected.
When you adjust the mixture lean you increase the exhaust gas temperature and begin to make nitogen oxides, a key pollutant that the oxy sensor tries to avoid by keeping out of the lean zone.
Too hot exhaust gas temp can be tough on valves and other parts of the engine.