Hello and thank you for allowing me the opportunity to assist you.
I would fight the claim. First and foremost, a diesel nozzle won't even fit in the gas tank of a car that uses unleaded gas. They are purposely made too large in order to prevent exactly this kind of mistake. So, a person would have had to try pretty hard to get diesel fuel in the tank, and mistakes are unlikely.
If I were you, I would write the rental car company a letter and deny ever using diesel fuel. You may wish to point out the size of the nozzle, as mentioned above, and that you only used regular unleaded gas. The rental company's recourse would be to sue you. And in a lawsuit, they'd have to prove by a preponderance of the evidence that you put diesel fuel in the car. That would likely require expert testimony
from a mechanic who can testify about finding the diesel fuel in the tank, and testify that the damage to the vehicle was consistent with diesel fuel being in the tank, and most importantly, that it would have been impossible for diesel fuel to be in the tank unless you had put it there sometime after you picked up the car.
For $800, I seriously doubt the rental company will bother with a lawsuit.
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