If people are told they will have to buy a tank of fuel that's non-refundable if it's not used it's not "corrupt" surely ? I don't get a rebate if I check in to my hotel on a compulsory breakfast-included basis, then stay out all day and all night and crawl back in at noon the next day just in time for check-out (never !
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But if the tank of fuel option is chargeable at full pump prices or higher, anyone with common sense can work out that the likelihood is they'll end up handing back unused fuel and so the effective cost of their rental will be an unspecified amount higher than the rental price, up to the cost of the tank of fuel. It's just hire revenue by the back door for the company. It's a problem where companies don't quote specific figures for the cost of a tank or the pricing mechanism at booking time. Some do, some don't.
Personally I hate discount car hire booking sites because most don't reveal who the hire company is until you have paid - biggest bug bear.
There's often a high risk of 'possible extras' and sometimes you can't get a figure/price for the cost of an inclusive tank (or out-of-hours fees and the like) before you've booked/paid without calling a premium phone line, even if you know you'll be buying a whole tank (for example) in principle. Companies like Goldcar in Malaga charge far more than the pump price for a tank (or did last year anyway). All of that rubbish is not playing ball and best avoided by just going somewhere with clear inclusive transparent pricing if you can and it's in your budget. Still, in Europe usually full-to-full is best, as long as you don't mind leaving time to fill the tank before you give the car back. imho. That way you only pay for what you use.
The tank-inclusive fee thing is common in the USA in places like Florida, and is usually extremely good value there. I rented a car from Alamo in Orlando for a day on a layover, paid for an entire inclusive tank of fuel, drove 10 miles round trip and gave it back. Worked out less than half the price of a round trip taxi (the alternative option). I felt no pressure to use up the tank of fuel just because I'd been charged for it .. was still quids in overall. It all depends on how you look at things
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To me, the major companies are the international ones or their local franchises (Hertz, Avis, National, Budget etc), which all have in-terminal depots at the airport. They all operate full-to-full policy on fuel as far as I'm aware. It's only the local firms which do the 'pay-for-a-tank' thing.