I am having some real problems renting at US locations as a Sixt Diamond member. You can read my experience in San Jose where I just gave up and cancelled my reservation while at the facility.

I booked a one-way rental, about 3 months in advance, at Sixt LA and wanted a nice car. I booked the Range Rover HSE, then received an email from them a few days later that it isn't available. Then I booked the Mercedes G-Wagon, again received an email it isn't available. Then I booked the Merc S-Class, again got the e-mail not available. How can none of these cars be available 4 months in advance?

I emailed them asking what is available. They got back to me that the Range Rover Sport was available for $750 for 2-days. I book it knowing as a Diamond I am guaranteed a 1-class upgrade, or at least a 1-class category price downgrade.

I show up for my rental and ask about the guaranteed Diamond upgrade. They tell me that this is a "specialty car" and is not upgradeable, whether Diamond or not. Really?! I don't recall seeing such terms in Sixt's Diamond upgrade documentation. I then asked about being charged a class lower since they won't upgrade. They said no, one can't be upgraded to the Range Rover, so if I booked a lower class I would not be able to be upgraded to the Range Rover. Basically, take the Range Rover at full price and receive no benefit for being Diamond, or cancel the reservation. This is the overall same result I got at San Jose. So, again, I canceled my reservation.

It doesn't appear that Diamond status has any relevance here in the US.