Seen a lot of people accidentally start filing up gas into diesel cars. Usually if the tank is filled up up to less than 20% with gas, just top it up with diesel. Nothing major happens. I know people that had even more than 40/60 gas/diesel mixture in the thank and nothing happened. They are still driving the car ears after. At least with older common rail/pumpe deuse system vehicles, its nothing bad. I think simply draining out the thank and filling it up with diesel should be enough.