My policy is when I'm riding in sombody else's car over a long distance, I pay for all of the gas. I do that because I think in terms of total operating cost and the car owner is already shouldering all the depreciation, etc.
That said, MOST people only think in terms of "gas" for operating expense of a car, so people generally don't offer me any compensation when we take my Tesla but occasionally I'll get a coffee out of it.