Even the most luxurious of internal combustion engines have nothing on electric powertrains when it comes to smoothness and quietness. No amount of engineering can erase the simple fact that an internal combustion engine relies on heavy reciprocating parts and thousands of tiny explosions each minute to generate its thrust. The movement of electrons through wire coils is comparatively quiet.

That makes electric powertrains just about perfect for opulent luxury cruisers like this classic 1970 Rolls-Royce Silver Shadow, which has been given a new lease on life thanks to a Tesla powertrain swap from the experts at Shift EV.

We can't possibly know how late country music legend Johnny Cash, this Rolls-Royce's former owner, would feel about the conversion, but it's tough to find fault with.

At 50 years and 130,000 miles old, the Rolls-Royce was in dire need of a complete mechanical restoration. But rather than go through the expensive hassle of simply rebuilding the car as it might have been when it left the factory, Shift EV took a brand new Tesla Model S with a 75-kWh battery pack, stripped it down to its bare essentials, and installed those components underneath the Tolls-Royce's skin. The desire to keep the exterior and cabin as original as possible led to some interesting solutions.

Here's an example: the Model S's digital displays, including the big tablet-like main screen that typically constitutes the EV's center stack, ended up installed in the trunk, so as to not require the removal of the classic Rolls-Royce center stack and gauge cluster. Another example: Shift EV had to alter the battery pack's housing to remove the lump formed into the top so that it would lay flat underneath the factory Rolls-Royce floor.

It's a fascinating custom build, with a list of modifications that almost overshadows its ownership history. Here's to another 130,000 miles on the road.