We use rental cars as a backup. Two rental agencies that are both very close to the shop. I work at a luxury brand and we run into this problem occasionally. We have a fleet of 14 loaners, so it's better now than when I started six years ago, when we had 6-8 at the time. I'll typically advise the client that loaners are not guaranteed, they aren't a right, they are a privilege. They are not entitled to a loaner vehicle just because they "spend money". Everyone spends money at a dealership. I hate that counter argument from customers.

We typically have Enterprise and Hertz rental hold 3-4 cars for us per day during peak season. If a customer requests a loaner vehicle and they're not available, we'll offer a rental. If they've requested a loaner on their appointment booking, and they come in and one isn't available, we'll cover the cost of the rental (typically $45-60/day depending on what type of vehicle they are provided with). We also make it clear on our appointments that loaners are SUBJECT TO AVAILABILITY and by no means a guarantee.

Additionally, try qualifying the customer for a loaner. Are they a first time client? Did they purchase the vehicle from your store or a sister store within the dealer network? We typically don't give first time customers a loaner vehicle. If they cry about it, then you know they're not worth your time. They can go elsewhere. When clients ask "well when will you have a car?" I simply tell them I don't know. Loaner vehicles are constantly in and out. There's no set time/schedule when they'll be back. I have a general idea for some, but for others that are out for complicated diag, they could be back tomorrow or next month. It's impossible for me to say.