The Audi E-tron Spyder concept is not just an open-top version of the E-tron sports coupe concept. The hardtop is all-electric, while this one is a plug-in hybrid that uses a turbocharged, mid-mounted diesel engine to power the rear wheels while the front wheels are each driven by their own electric motor. The funny, but good, thing about diesel and electric motors is that they’re gloriously torquey, and they deliver their shove at low, low revs. The E-tron Spyder has a total of 738 lb-ft, which is a huge pile of torque for a lightweight carbon-fiber body with a bespoke aluminum spaceframe. The horsepower count is 401. When in electric-only mode, the car can cover 30 miles, or cruise at 60 mph for a shorter distance. Harness both power sources and the sprint to 60 mph takes just 4.4 seconds. There is no mechanical connection between the front and rear axles, just a bunch of wires and computing power, and the combustion engine isn’t co-opted into charging the lithium-ion batteries. No firm plans have yet been announced to make the E-tron Spyder into any kind of production car, limited or otherwise. But the technology for a lightweight diesel plug-in hybrid from Audi already exists and is being applied to the tarmac, returning 108 mpg as it does so. There’s no reason why some future A4 (as a hypothetical example) couldn’t have it.