Toyota sold a van here in Asia that was notorious for being a gas guzzler. We called it the Revo, but the more common name is the Kijang. Just 2 liters, but it went down to 10 mpg in city traffic at times (without idling for hours at the stoplight). When the shop fitted it with an LPG kit, they dyno'd the thing. Imagine our shock when this fuel-injected, two-liter four-banger showed AFRs of 10:1...
at idle.
Thus, when the LPG kit was tuned, it saw a 30-40% fuel economy increase instead of the typical 10% decrease.
Worst case AFRs I have seen from stock gasoline vehicles are in the 9-10:1 range. Any lower and you'll be spewing fuel droplets out the tailpipe.