The early evidence is that they could be, though most of the savings come from the interconnections and business models that autonomous vehicles make possible, rather than the cars themselves.
A computer drives better than a human being in almost every situation you can imagine. So they won’t ever be in the wrong gear, or over-revving a hill start. They will accelerate and brake smoothly, and will thus be more efficient drivers. At higher speeds they could drive closer together and reduce energy use through improved aerodynamics, something known as platooning. This would be true of both electric and petrol autonomous vehicles, though almost everyone thinks that electric vehicles will predominate.
Savings come through coordination to reduce traffic jams. Because driverless cars will be in communication with each other, news of any delays or obstructions would be relayed through the network of vehicles on the road. Cars would know to find an alternative route, or slow down to avoid a build-up of traffic. This would save the wasted energy spent queuing.
A driverless car doesn’t spend time looking for somewhere to park. That might sound trivial, but it depends where you live. Drivers in London spend an average of eight minutes at the end of each journey looking for somewhere to park. That’s a lot of wasted time and energy every year, all contributing to air pollution in the city. Driverless cars could help in two ways here. First, in a connected city the car would know where the free parking spaces are. It would drop you off and then go and park itself, and come back for you later. And secondly, if a car is part of a club or taxi service, it wouldn’t need to park at all.
Driverless cars mean fewer cars on the roads. At least, they should in theory. To really unlock the benefits of autonomous cars you need to drop the idea of personal ownership and consider them a form of public transport, with fleets of shared cars. Not everyone is ready for that, but it makes sense in cities. Cars would pick up and drop off passengers, and move on to the next journey rather than parking up and sitting un-used for 95% of the day. That means that each car on the road is being used, and we need fewer cars overall.