Girl: Aw, well thanks for the ride Brian-
*she begins to exit the car*
Brian: Hey- hey, hey! Not so fast, its ass gas or grass in this car, bitch. And it seems I've got a full tank and plenty of the fine erb, so uhhh... *nods head, indicating genitals*
Meaning you wish to take advantage of the situation by getting a good fuck, some bucks for the gas you´re gonna need or at least something to get a buzz from.
(Maybe one of you might describe it better, as I´m by no means a native speaker!)
ass gas or grass ?
Nobody rides for free, you know?!"