Dfink wrote,
Of course in an airplane you are under pretty much full load constantly
Yes, and a 65 mph cruise at 15% throttle in a car is a much lower load. If you are a pilot and understand LOP ( lean of peak) fueling, it should be easy to understand that a road car under lower load can run a lean of stoich mixture.
There are some good articles on fueling on this site:
http://www.avweb.com/news/pelican/pelicans_perch_84_mixture_cht_194816-1.html
which recommends against using CHT valvues for fueling.
There are also good threads on the LM-1 Innovate site about lean cruise fueling using +15 AFR's