Without knowing the test conditions, torque and other performance results, the lone number of fuel milage is worthless. Stirling engines are usually big, heavy suckers, better for fixed installation than vehicles.
Not to mention with all the mpg test fraud going around these days.
meh, totally false. There have been prototype cars before that use stirling engines, the main flaw has been slow start times and poor speed change (it takes a while to heat up, and for changes in heat to take effect). Making it a hybrid solves both of those problems!
Probably the (non-CO2) emissions. Most engines on the market have to optimize both for fuel efficiency and minimization of pollutants. As you might remember from the VW scandal, you can improve fuel efficiency at the cost of greater NOx emissions.
Also, they need to be big and strong enough to pass crash tests, which further reduces fuel efficiency. I don't think this car would pass those either.
Presumably the big different is that 200 years ago you did not have decent batteries and electric motors.
I'm really surprised by how little attention this key fact is getting. The best engine for a range extender duty is very different from the best engine for direct drive, internal combustion or not.
but is it worth the extra weight and complexity of a second engine and fuel source?
The answer at present appears to be "yes, unless your use case allows frequent recharging" (such as an EV commuter car/grocery getter, alternate transportation available for long trips, no likely situation where unplanned long trip would be necessary) or there's large financial incentives to ditch the engine (i.e. taxpayers paying people for the inconvenience).