How the Reliability Index works

The data

The DVSA publishes every MOT test in Great Britain, anonymised, as open data. We use the three latest years (2022 to 2024): 104,887,872 tests that meet the rules below.

Which tests count

• Class 4 vehicles: cars, people carriers, campervans and vans up to 3,000kg.
• Normal tests only: retests after a fail are left out, so a car counts once per MOT.
• Results Pass, Fail and PRS (passed after a repair during the test). PRS counts as a first-time fail, the industry standard.
• Vehicles aged 3 to 25 at the test, with a valid first-use date.

Like for like: the age and mileage adjustment

Older and higher-mileage cars fail more often, so raw fail rates flatter young, low-mileage models. For every test we look up the national fail rate for cars of the same age and the same 20,000-mile band, and add those up to get the fails we'd expect for that model.

A model's vs average figure is its actual fails against that expectation: −20% means 20% fewer fails than cars of the same age and mileage. Areas of failure (brakes, suspension and so on) use the same method.

Names, thresholds and limits

• Model names are matched to one base model per car ("CORSA SE PREMIUM" counts as Corsa) with one published alias table. A model needs 5,000+ tests for its own page; a model year 1,000+; a point on a chart 200+.
• MOT results measure roadworthiness on the day, not breakdowns or running costs. How a car is looked after matters as much as the model.
• The index covers 551 models and is rebuilt when the DVSA publishes a new year.