Auto Loan ABS and Loss Timing Curves, Explained

Two auto loan pools can have the same total expected lifetime loss rate — say, 3% of original balance — and still need very different amounts of credit enhancement in their first two years, because losses on auto loans don't arrive evenly. They follow a loss timing curve: low in the first few months, rising to a peak around 18 to 30 months into the loan's life, then tapering off in later years as the pool of loans that are going to default has mostly already done so.

A pool's total lifetime loss rate tells you how much a deal has to absorb eventually. Its loss timing curve tells you when — and a deal has to hold enough credit enhancement to survive the peak loss period, not just the eventual average.

Why losses hump in the middle

Very new auto loans rarely default in their first few months — a borrower who just qualified for financing and made a down payment is unlikely to stop paying almost immediately. As loans season past a year, though, the effects of job loss, divorce, or simple over-extension start showing up, and defaults climb. By years three and four, borrowers who were always going to struggle have mostly already defaulted, and the loans still performing are a self-selected, better-behaved group — so the default rate declines even as the pool ages further. The result, plotted against loan age, is a hump: low, then a rising peak, then a long decline.

Auto loan losses follow a predictable hump: low early, peaking between roughly 18 and 30 months of loan age, then declining as riskier loans have already defaulted out of the pool.

Worked example

A $500 million auto loan pool is expected to realize 3.0% lifetime losses ($15 million) over its life. The loss timing curve says roughly 55% of that lifetime loss will occur in months 12 through 30 — about $8.25 million concentrated in that 18-month window. A deal structured with a level, non-declining credit enhancement sized only to the 3.0% lifetime average could be badly under-protected during that peak window if enhancement hasn't been front-loaded to match it; rating agencies size initial subordination and reserve accounts to withstand the peak-period loss rate, not merely the smoothed lifetime average.

What this means in practice

Deal structures respond to the hump by front-loading credit enhancement — starting with a reserve account and subordination level sized for the worst 18-month stretch — and often allowing subordination to "step down" later in the deal's life once the pool has passed its peak-loss window and performance has proven out, releasing some excess credit enhancement back to the sponsor.

A deal's average annual loss rate can look comfortably low while the deal is still in its peak-loss window and under real stress — judging a young auto ABS deal by its lifetime-average assumptions, rather than by where it sits on the loss timing curve, understates how much stress it's actually facing right now.

Auto Loan ABS and Loss Timing Curves

Why losses hump in the middle

Worked example

What this means in practice

Related concepts

Practice in interviews

Further reading