I was told that if you borrow a car from a friend, it's courteous to return it washed, and with a full tank of gas (even if the car was dirty and had an empty tank when you borrowed it). Is there any problem here of ribis/usury/interest on a loan?
Ribis is not on an Item that you are going to to return the very same item itself, so essential Ribis is only on Loans of money or things of Monetary value, not of items where you return the actual item itself.
To illustrate for example borrowing eggs(this is Monetary Value) and returning more eggs since there you are lending the value of the item not the actual eggs it is not allowed again monetary value=Ribis. That is even if the price of sugar goes up from when you borrowed it the problem of Ribis applies. so you are allowed to borrow the car and fill the tank.
For a more in Depth treatment see Here:
Even if the borrower did not return more gas than he received there would still be a question concerning returning the same amount of gas as was "borrowed", since gas is a consumable item and when someone borrows a consumable item, even if it is to return the same amount there is an issue called "seah b'seah". This is a rabbinic prohibition based on the concern that the value of the item borrowed may increase between the time of borrowing and the time of returning. Thus if a price increase occurred, then even returning the same quantity as was taken would be prohibited because of the increase of value. Based on this, the Chachamim prohibited borrowing any consumable item and returning the same amount as was taken, even if no price increase occurs, because of a concern that one may return the same amount even if a price increase did occur. Thus, if someone borrowed (or rented) a car, a question arises as to why he may return the car even with the same amount as gas as was in the car at the time of borrowing or renting it, (as is the practice with all car rental companies). Why does this not a violate the concept of "seah b'seah"?