i run my '90 RRC 3.9 on just petrol.
financially crippling.
i only use petrol in my 3.9 rrc.
I suppose i don't do big miles in it most of the time and £30.00/week is about what i use.
On a run consumption varies enormously but i reckon on anything from 18mpg up to 23mpg. Fooling around in the mud/snow or just booting it everywhere i reckon on as little as 6mpg (but not often!!) but as an average i would say 12mpg.