LPG?

[Kezzieboy]

Look around for cars that are already converted, as obviously that's where the biggest savings can be had.

Dad had a 328i estate on LPG for a while, was noticeably slower on LPG, and the fuel economy was worse, but they're small compromises for what you get in return. He eventually got rid because the price of LPG didn't settle down again after a fuel price spike, so the difference in cost to petrol wasn't enough, and a diesel became a far cheaper option.

Also, as I guess all of them are dual fuel, if you want to go on a proper blast, you can just flip the switch back to petrol. Dad used to literally flip the switch for overtaking.