Methodology
Where the data comes from
Every figure is derived from the US National Highway Traffic Safety Administration's public APIs at api.nhtsa.gov: the owner complaint database and the recall campaign database. Data is refreshed daily. We store no personal information from complaint records.
The severity index
Raw complaint counts track sales volume closely, since a best-selling sedan collects more filings than a rare coupe. The severity index measures the composition of those complaints instead, per 100 filings:
| Event in a complaint | Weight |
|---|---|
| Crash reported | 4 |
| Fire reported | 6 |
| Injury reported | 8 |
| Fatality reported | 40 |
Score = (4xcrashes + 6xfires + 8xinjuries + 40xdeaths) / complaints x 100. The weights are our own, set so that a single fatality report outweighs a large number of non-injury reports. They are not an NHTSA standard.
Adjusting for sample size
That rate on its own is unstable when a vehicle has few complaints. One serious report out of 40 filings produces a very high score that reflects the small sample rather than the vehicle. Before publishing, each score is therefore blended with the fleet average using a prior worth 100 complaints:
Published index = (weighted events + 100 x fleet rate) / (complaints + 100) x 100.
A vehicle with thousands of complaints is barely affected. A vehicle with 40 sits close to the fleet average unless its record is genuinely extreme. Without this step the highest-scoring vehicle on the site was one with exactly 40 complaints on file.
What Low, Medium and High mean
The bands are relative. Each vehicle is compared against the rest of the fleet on file, using the lower and upper quartiles of the dataset as measured on 2026-08-14. The cut-points are then held fixed so that a given severity index means the same thing from one week to the next:
| Band | Severity index | Meaning |
|---|---|---|
| Low | below 46 | cleanest quarter of tracked vehicles |
| Medium | 46 to 80 | middle half |
| High | 80 and above | worst quarter of tracked vehicles |
The middle half of tracked vehicles score between 46 and 80. Because the bands were set at the quartiles, roughly a quarter of vehicles fall in the High band. The band states where a vehicle sits relative to others on file.
Known limits
- Complaints are unverified consumer reports, not confirmed defects.
- Counts are not normalised by units sold or vehicle age.
- Media coverage and class actions measurably increase filing rates.
- A vehicle with few complaints may simply be rare.