Trueslayer;
That '100 MPG Cadillac' story is just another urban legend, which has surfaced with every model of car available since the 1960s. Having lived in more than 20 states, and having heard the same story with [fill in brand and model here] car(s), it has been most remarkable that the exact same story has been told over and over, even on the Internet, with only the brand substituted, year after year. Don't buy it.
For the truth, one only needs to check the physics: if ALL of the heat energy of a burned gallon of gasoline (in a 20% oxygen atmosphere) were applied to the rear wheels of a 3,600 lb car, it would travel about 65 miles on a perfectly flat road (assuming no tire losses). Witnessing the fact that the latest EFI-controlled vehicles (at about 1800 lbs) are now approaching this figure shows the mastery of today's combustion control engineers!
Just one comment about using bike carbs, which I know very, very well from 30 years of racing/driving/building with them, they tend to run below their apparent size if the vacuum is constant. By this, I mean that they are designed to be used in a pulsing-vacuum environment. Some have 'constant-velocity' pistons of one type or another that smooth out the pulsing physics somewhat, but by and large, they act as if their venturi diameter 'shrinks' as the vacuum through them becomes more constant. For this reason, they are usually larger than a chart might estimate them to be, unless the irregular vacuum is taken into account. A good rule-of-thumb for single-cylinder design is to use a carb that has the same venturi as the intake valve's hole diameter. Going smaller in the venturi will generate better low-end response, while going larger will increase top-end HP, but at the cost of poorer fuel economy because of poor atomization at low speeds.
