Dave Akhurst's LPG Greenfly.

Via: the biker gene

"Dave Akhurst is a braver man than you or I. I feel confident in saying this because he rides around the countryside on the Greenfly - a heavily modified XT500 Yamaha that runs on Liquid Petroleum Gas (LPG).

The benefits of running vehicles on LPG are reasonably cheap running costs due to lower government taxes on LPG than unleaded petrol in most places, it has a high octane rating (108RON) and burns quite cleanly. On the flipside, you tend to get significantly less mileage per litre on a tank full of LPG than you would on petrol.

The 500cc single still uses a kick-start, although Akhurst would put an electric leg on any production variant. It makes a lazy 30-odd horsepower and Akhurst figures he gets around 70mpg - compared to roughly 50mpg from a standard late 70s XT."