Filling up the tank of a borrowed car

Question:

My mother in law lent me her car for the month I just filled up the entire tank before returning it.
Is that somewhat assur because of ribbis?
Thank you

That was the correct thing to do, because that is simple hakaras hatov fopr the fact that she lent you her car for free. There is no issue of ribbis here because ribbis only applies to lending money or something that you borrow one thing and you use it up and return something else in its place, such as money, eggs etc. When you borrow something and return the actual item itself, it isn’t ribbis and you may even pay the person money to use it, such as when we rent a car and pay the owner money as rent. It isn’t ribbis, and permitted.

Best Wishes

Sources:

Baba Metzia 69b, Y:D 176-2.

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1. Chaim Mendelson says:

The issue seems to revolve around the question, if the tank of gas is considered as part and parcel of the car. Or do we perhaps say that the car is a car, and it happens to have a tank of gas in it, which dispenses the gas needed to operate the car.

The questioner seemed to entertain the possibility of the latter, which would make it a Ribbis issue, because even though they borrowed a car and returned a car, they also borrowed a partial tank of gas (with x gallons/liters), and returned a full tank of gas (with x+y gallons/liters). Which would analogous to returning a dozen eggs, when I only borrowed, say, 9 eggs.

In your answer, you assume that the tank of gas is considered an integral part of the car, and not a separate entity.

Can you please provide a source for this assumption?

1. Rabbonim of the Beis Hora'a says:

R’ Yisroel Reisman in The laws of ribbis 1-10 says this idea, that even though one may argue that the gas is a separate loan and subject to ribbis, however it is apparent that the extra gas returned is for the lending of the car, not the lending of the gas. He quotes Mishnas Ribbis 6 ftnt. 5. See also bring Chayei Halevi 2-53 that he should specifically say that it is for the lending of the car.