How far from your shop to the nearest gas station? Perhaps your test drive needs to be just long enough to leave barely enough for the customer to get to a gas station, after picking up the car from your shop.
KRAMER: Listen to me. When that car rolls into that dealership, and that tank is bone dry, I want you to be there with me when everyone says, "Kramer and that other guy, oh, they went further to the left of the slash than anyone ever dreamed!"