Insurers sometimes produce diminished value figures using a formula rather than market data about your specific vehicle. The most widely known version is usually described as 17c, and its output is distinctive: a number that feels arbitrarily small relative to a serious repair, particularly on a car with ordinary mileage.
It is worth understanding the structure, because knowing which step produced the number tells you which evidence is worth sending back. We implement 17c in our own calculator purely as a comparison, so you can see the two approaches side by side.
The three steps
The formula runs in sequence, and each step can only reduce the result. Understanding that sequence is most of what you need.
- Start with a base value for the vehicle and take a fixed percentage of it — commonly ten percent — as a ceiling on the possible loss.
- Multiply that ceiling by a damage modifier, a factor reflecting the severity of the damage, usually stepped from none through severe.
- Multiply the result by a mileage modifier, a factor that decreases as odometer readings rise and reaches zero above a threshold.
A worked example
Take a vehicle with a base value of $32,400 that sustained a structural repair, with 45,000 miles on the odometer. The ten percent ceiling is $3,240. A damage modifier for major damage — 0.75 in the version we implement — brings that to $2,430. A mileage modifier of 0.6 for that odometer band brings it to $1,458.
Note what happened there. A vehicle with documented structural repair produced a figure under $1,500, and the two modifier steps removed more than half the ceiling. Note also what never entered the calculation: the actual repair cost, what comparable vehicles in that market are listed at, whether panels were replaced, whether driver-assistance sensors needed recalibration, and what the vehicle's history report now says.
Why the mileage step draws the most criticism
The mileage modifier is the step that most often produces a result drivers find hard to accept, and the reason is structural rather than a matter of opinion. The modifier declines in bands as mileage rises and, in commonly described versions, reaches zero above roughly 100,000 miles. At that point the formula asserts that a vehicle cannot suffer any diminished value at all, no matter what happened to it.
There is a defensible idea buried in there: higher-mileage vehicles are already discounted by buyers, so an accident record may move the price less in absolute terms. The problem is the shape of the implementation. A stepped multiplier that hits exactly zero is a much stronger claim than the underlying reasoning supports, and it does not vary with the severity of the repair or with what the local market actually does.
Which inputs are worth questioning
Arguing about whether the formula is a legitimate methodology rarely moves a claim. Auditing its inputs sometimes does, because an input that is wrong about your vehicle is a concrete, answerable error rather than a difference of opinion.
- The base value: does it match your vehicle's actual year, trim, options, and condition, and does it match your documented pre-accident value? If it starts low, everything downstream is low.
- The damage characterisation: does the severity band match your final repair invoice? A repair involving structural work, replaced panels, and sensor calibration characterised as moderate is a factual mismatch you can document.
- Whether the vehicle was inspected at all, or whether the figure came from a desk review of a few data fields.
- Whether any comparable vehicles were considered, and if so, whether they are genuinely comparable to yours.
How to respond
Respond in writing, and respond with evidence about your specific vehicle rather than with a critique of formulas. The most effective reply takes each stated element of the insurer's figure in turn and attaches the document that addresses it: the invoice against the severity characterisation, your valuation support against the base value, and comparable listings against the formula as a whole.
Comparable listings matter most here because they answer a question the formula never asked. If you can show what similar vehicles without accident history are listed at, and what similar vehicles with disclosed accident history are listed at, you have put an actual market spread in front of the reader. That is a different kind of submission from an argument that a multiplier is unfair.
What we do instead, and why we still show you 17c
Our own estimate does not use the 17c structure. It starts from the repair-cost-to-value ratio to set a severity band, adjusts for documented repair characteristics such as structural work and sensor calibration, then applies multipliers for vehicle age, mileage, prior history, title status, and whether you have supplied usable comparable listings. Every step is shown in the report, and the coefficients are published.
We still calculate a 17c figure alongside it, for one reason: if you receive a formula-based response, you need to see the two numbers next to each other to understand the size of the gap and what is causing it. Showing it is a comparison, not an endorsement, and it is not a statement about what any insurer is required to do.
Questions
Is 17c a law or an official standard?
It is a calculation method, not a legal standard, and we do not publish claims about its legal status in any state — that is a legal question we will not guess at. What we can say is that it is a formula an insurer may choose to apply, and that its inputs can be audited against your documentation.
The insurer's figure is zero because of my mileage. Is that the end of it?
A formula output of zero is one method's result, not a finding about your vehicle. You can respond in writing with market evidence for your specific car. Whether it is worth your time depends on the size of the documented gap, which is what our estimate is for.
Why does your estimate differ so much from the 17c figure?
Because they measure different things. 17c applies fixed modifiers to a base value and ignores repair cost and market comparables entirely. Our estimate starts from the repair-to-value ratio, adjusts for documented repair characteristics, and takes account of whether you have supplied usable comparable listings. Neither is a licensed appraisal.