The next time you borrow a friend or relative's car, fill it up with gas for them before you give it back. Or if you are on a road trip with your friends and you're taking their car, when you stop to fill it up, offer to pay for the gas. This is a thoughtful and generous thing to do.
You are letting that person know how grateful you are to them for letting you borrow their car. This is a simple way to go above and beyond and to bless someone with your thoughtfulness.
Even if the vehicle wasn't full of gas at the time that you borrowed it, go ahead and fill the tank completely before you give it back. That way you're not just replacing the gas you used but also giving your friend some extra gas as a "thank you" for letting you borrow it.В You can also do this for someone out of the blue--you don't have to borrow someone's car to pay for their gas. For example if you notice your spouse's car is low on gas, go take it out and fill it up for them so they don't have to leave early the next morning to fill it on the way to work.
If you don't have time to fill it up before returning it, be sure to give your friend a gift certificate to a local gas station within a week of borrowing their car. Keep in mind, too, that filling someone's car doesn't have to be done just as you're borrowing it. If you know your friend is leaving for a big trip the next day offer to give them time to pack while you go fill their car up. Or, offer to drop them off at the airport and pick them up when they return, saving them from paying for parking for the duration of their trip.