Every model is placed into an insurance group before it reaches the market, and that placement affects what owners pay for the life of the car. The rating is derived from measured properties rather than from the vehicle's price.
What the grouping is for
Insurers need a consistent way to compare thousands of variants, and assessing each individually would be impractical.
A grouping system provides a common scale that every insurer can use as a starting point, applying its own loadings afterwards.
The rating therefore describes the vehicle rather than the driver, and it is only one input into the final price.
How repair cost is measured
The largest component is what it costs to put damage right, assessed from standard repair times and from the manufacturer's parts prices.
Test crashes at low speed are used to establish what actually breaks in a common impact and how long it takes to replace.
A car whose front bumper can be removed quickly and whose parts are modestly priced rates better than one requiring extensive dismantling.
Why performance still matters
Acceleration and top speed correlate with the severity of collisions, so both feed into the assessment independently of repair cost.
Braking performance is weighed on the other side, since a car that stops in a shorter distance is less likely to be involved in an impact at all.
This is why two cars with identical bodywork and different engines can sit several groups apart.
What security contributes
Theft resistance is assessed directly, covering locking systems, immobiliser design, alarm specification and how easily a vehicle can be taken without its key.
Models targeted by particular theft methods can be reassessed after they reach the market, which is why ratings occasionally move for cars already in service.
Tracking systems and improved key security can pull a rating back down, and manufacturers respond to a poor placement because it affects sales.
Why the same car varies between insurers
The group is a starting point, and each insurer then applies its own claims experience for that model, which may differ from the industry view.
Driver factors, location, mileage and the use to which the car is put all modify the price substantially from that base.
Comparing group ratings between two cars is therefore a reasonable way to anticipate relative cost, but it will not predict what any individual quotation turns out to be.