Here are two places for you to start:
The first one is a report from Doble, in which he talks about achieving 14% efficiency at 25 mph, and 16% peak.
Dobles were the most advanced steam cars of all time. In the late '20s, whcn a Ford cost $260, a Doble cost $10,000 or more.
Steam technology on a small scale hasn't progressed very much since. The Doble was a work of genius and very advanced. Turbines are out for cars. You need positive-displacement engines, like IC piston engines. Workable steam engines are complex; boilers are maintenance headaches; and so on.
If you want to see what has developed since, there is plenty on the Web. But watch out for the enthusiasts' sites. They usually don't tell you things like how you'll spend the weekend descaling your boiler, or the fact that it takes three or four times as much fuel as an IC engine.