Why not use the 50MPG genset in a regular gas-powered car?
50MPG takes into account the availability of batteries to store charges from regenerative breaking and such. The Volt genset may also shut off when electric demand is low or non-existent, and rely on the remaining low charge to supply power to low-drain devices. In any event, “50MPG” is the best case scenario, as is “40-mile all-electric-range”.