Chris Bryant is a Bloomberg Gadfly columnist covering industrial companies. He previously worked for the Financial Times.

2017 is shaping up to be an excellent year to purchase a used car. That's great news for consumers -- but a big worry for auto manufacturers, rental companies and auto finance groups.

This chart explains why:

Lease Bubble
A pile-up of nearly new cars returning off lease is set to cause trouble for the U.S. car industry
Source: Manheim Used Car Market Report
Data for 2016-2018 are an estimate

Thanks in part to low interest rates, leasing has become an increasingly popular way to drive away a new car. It accounts for almost a third of all new car transactions in the U.S. and it's also huge in the U.K., as I explained here. For BMW and Mercedes-Benz in particular, it's been a boon for sales.

Leasing Boom
Leasing is no longer just a luxury segment phenomenon. Low interest rates have helped.
J.D. Power and Associates

Typically a lease lasts about three years, after which the customer returns to the showroom for another vehicle -- which is when things could get difficult for the industry.

"There's going to be a lot of units coming back over the next several years," Ford Motor Co. warned last month. "They're going to get to levels that we have never seen on an absolute basis in the industry before".

In 2017,  about one million more off-lease vehicles will be available in the U.S. compared with 2015. That additional volume will put downward pressure on used car prices.  

If cars depreciate too quickly, consumers will be unwilling to pay high prices for new vehicles. High residual values also help to keep monthly lease payments low. In other words, if used car prices fall, the whole system comes unstuck: automakers' earnings will likely fall and car finance companies (often a subsidiary of the manufacturer) may have to book writedowns on the value of their leased assets.   

During the 2008 recession fewer people bought new cars, meaning there weren't so many used cars for sale in subsequent years. That's helped price to hold up fairly well, but there are signs that's all about to change.

Last month, Hertz Global Holdings Inc. warned of a big increase in depreciation rates for its rental fleet due to lower than expected resale values. It warned residual value pressures would continue into next year and that others in the industry "can't be immune to this factor." Its shares plunged.

Residual Risks
Hertz shares have tumbled over the past year. Larger than expected depreciation rates aren't helping

Industry-wide used car prices are falling most for compact and mid-sized cars, reflecting U.S. consumers' increasing preference for SUVs and trucks. 

Big is Best
Small cars are depreciating more quickly than large pickups and SUVs
Source: NADA Used Car Guide
Data show month-on-month change of wholesale used vehicles prices in October for cars up to eight years in age

Investors aren't oblivious to the risks. Although Donald Trump's election caused auto stocks to rally  -- due to a possible relaxation of costly fuel economy regulations and tax cuts that might spur auto purchases -- it's rare to find an automaker that trades on more than eight times estimated earnings (Fiat Chrysler fetches just five times). 

There's one big consolation, though: nearly-new vehicles should be going cheap next year.

This column does not necessarily reflect the opinion of Bloomberg LP and its owners.

To contact the author of this story:
Chris Bryant in Frankfurt at cbryant32@bloomberg.net

To contact the editor responsible for this story:
Edward Evans at eevans3@bloomberg.net