now I may be wrong but to use an air fuel ratioguage you have to have an O2 sensor...now, you can install one in your exhaust and thats what a lot of people do. I believe that a lot of the manufactures of the guages (like autometer) sell O2 sensor that you can install in your exhaust by drilling a hole in the pipe then welding a nut to the pipe. Hope I've helped.
Spork