The Hollywood stars slipped into the front seats of the £25,000 (£38,282) vehicle after officially unveiling it at a party in Old Billingsgate Market on the banks of the Thames.
The pair cuddled up next to each other and laughed like old friends as they explained the car's features, which include its own bespoke smartphone app and a hi-tech aluminium and carbon fibre plastic chassis.
Speaking at the launch, Miller revealed that she has become more conscious of environmental issues after having daughter Marlow with husband Tom Sturridge last July.
She said: "I've been involved in all sorts of different green charities for a decade now, and having had a daughter recently I feel incredibly responsible and we all have to make those choices for the next generation. It's very close to my heart."
Franco, meanwhile, stated the BMW i3 is "just cool", before assuring potential buyers that it is "not so big that you'll have problems parking".
Peaches Geldof, Wretch 32, Laura Whitmore, Sugababes singer Jade Ewen and Pixie Lott's model boyfriend Oliver Cheshire were among the other celebrities at the BMW event.