My nightly ramble…
I came across the Black Book on a dealer website, which led me to going to the Black Book website, which led me to reading all about it, and I found this page describing the Retention Index of theirs.
In their words, “The purpose of Black Book Wholesale Used Vehicle Retention Index is to provide an accurate and unbiased view of the strength of used vehicle wholesale market values. The index is calculated using Black Book’s published Wholesale Average value on 2- to 6- year old used vehicles, as percent of original typically-equipped MSRP. Black Book’s Wholesale Average is a benchmark value for used vehicles selling in the wholesale auctions with the vehicle quality in Average condition.”
Essentially, the strength of the used car market. When you look at the graph of their index over time, you could almost guess exactly what it is doing at a given time with basic economic knowledge. In other words, knowing the economic state of affairs at a time, you can guess relatively accurately how the index is doing. Pre- 2008 recession and post-2008 recession are a good example.
Anyhow, I thought this was fun. Useful for most people? No, but still fun 
https://www.blackbook.com/wp-content/uploads/2019/06/Black-Book-Retention-Index-2019_05.xlsx-Read-Only.pdf
I’m a bit confused about how they’re calculating the index:
The index is calculated using Black Book’s published Wholesale Average value on 2- to 6- year old used vehicles, as percent of original typically-equipped MSRP. Black Book’s Wholesale Average is a benchmark value for used vehicles selling in the wholesale auctions with the vehicle quality in Average condition. The index is weighted based on registration volume and adjusted for seasonality, vehicle age, mileage, condition, and inflation (MSRP). In short, this index measures the trend in percent retention from original MSRP of a typical 4-year old vehicle in the market.
So one might deduce that currently used cars are currently worth more than their original MSRP though that is obviously not the case. The “Black Book Wholesale Average” is obviously some sort of assumed rate of depreciation, we just don’t know what that rate is.
Personally I like to look at used car inventories, lease roll offs, and rental car fleet absorption to get a feel for where residual values are heading. Kind of an aside but if the used car market takes a turn in the next 18-months Hertz will likely be bankrupt.
Hertz may go bankrupt, but it isn’t b/c the used car market takes a nosedive.
There are about 20 different moving pieces that affect the industry as a whole, and big swings in 2-3 of them could cause some of the smaller RAC companies to go BK. Hertz is weirdly positioned; most expensive, best perks, newest fleet (last time I looked), most expensive airport locations…but IMHO the Achilles heel of other RAC companies (really, really, really crappy cars being 70-80% of their fleet) is not Hertz’s weakness.
Rideshare is really affecting RACs. But that has been partially offset by the decline of peak auto (which survived solely due to fleet sales last 2-3 years).
Not going down the rabbit hole here…as I said, there are 20 different variables.