[mnmarcus]

You can redesign a catalytic converter system to work with lean burn, but it'd add a lot of cost to the price of a car.

Thanks t vago,
Generally, could this additional be off set by the increased milage? In your opinion are the overall emissions worse for an aftermarket lean burn car than a regular?

From Air-fuel ratio - Wikipedia, the free encyclopedia

Air-fuel ratio (AFR) is the mass ratio of air to fuel present during combustion. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture (often abbreviated to stoich). AFR is an important measure for anti-pollution and performance tuning reasons. Lambda (λ) is an alternative way to represent AFR.