In message , Johny B Good
writes
The only other transmission method that occurs to me that might
compete with an all electric transmission is the use of hydraulic hub
motors in each wheel connected to a pump driven by a common motor[2].
A variable delivery swash plate type pump can provide a variomatic
auto transmission system with no need for a seperate clutch. I haven't
seen any comparative data on such a scheme though.

I think the transmission losses in a hydraulic system are on a par
with those in a conventional mechanical system. The costs are likely
to be considerably higher to produce an all hydraulic transmission
(even in mass production) for something as humble as the family car so
even assuming a similar transmission efficiency between the two
systems, it's not going to happen in anything other than specialised
vehicles (plough pulling tractors and Space Shuttle Transporters)
where the high precision of control outweighs the extra cost.

International Harvester brought out a tractor with just this system,
around 1975. I drove one here on trial. Bl--dy noisy!
--
Tim Lamb