Why we will never say "guaranteed to fit"
Three words that are worth less than nothing, and what we do instead.
· 683 words
Every parts site wants to tell you something will fit. It is the single most useful thing a parts site could say, which is exactly why the claim gets made far more often than it can be supported.
We are not going to make it. Here is the reasoning, and what we do instead.
The scale of the problem
Industry reporting on the auto parts aftermarket puts fitment errors behind up to 86% of returns, with as many as 30% of auto parts returned due to incorrect matches. The named causes are worth reading carefully because each one applies directly to the cars we cover:
- VIN decoders miss regional variants and mid-year production changes. Half the platforms in our catalogue exist in both Japan-market and US or European versions, with different steering sides and different parts. - Buyers mis-use or skip year-make-model dropdowns. Understandable - an S14 zenki and an S14 kouki look like the same car in a dropdown and do not share all their front-end parts. - A part fits technically, but not once aftermarket modifications are involved. This is the one that matters most for drift, and it is the reason our compatibility engine evaluates against the contents of your build rather than against your chassis. Fit an angle kit and a wheel offset that cleared perfectly now fouls a control arm.
What a broken guarantee looks like
Two documented complaints make the point better than any argument. One customer ordered a muffler under a "100% fitment guaranteed" badge; the installer could not fit it, and the return was then refused on the grounds that the part had been "installed". Another bought a CV axle after an agent confirmed fitment against their VIN by chat. It did not fit.
In both cases the customer did everything right. The guarantee was the problem, not the process - because a guarantee transfers confidence without transferring information. It tells you to stop checking.
The principle we took from the research
The clearest statement in all of our source material was about data quality: *accuracy is bounded by data quality; confirm fitment only where data supports it, and flag ambiguity instead of guessing.*
That is a design brief. So:
Confidence levels, on everything
Every compatibility rule and every part in our catalogue carries one of three labels.
Verified - we or a cited source confirmed this on this specific chassis. The source is on the page.
Community-reported - owners-group or forum consensus, linked. Widely held, not independently confirmed by us.
General principle - true in general, from physics or common practice. Measure your own car.
The labels are shown as words, not just colours, and they appear next to the claim rather than in a footnote. About a third of our compatibility rules are "general principle", and we would rather say so than round them up.
What we say instead of a guarantee
"This offset window commonly clears on a standard-bodied example of this chassis. It is a starting point, not a guarantee - ride height, camber, tire profile, which kit you fitted and whether the arches have been rolled all move the answer. Test with a spacer on wheels you already own before buying a set."
That is longer, less satisfying, and true.
The commercial argument, since it is fair to ask
Saying "guaranteed to fit" would sell more parts. Our revenue comes partly from affiliate commission, so we have an incentive to say it.
The counter-argument is simple: a planner whose fitment claims turn out to be wrong is worth nothing, and the complaints above are what the end of that road looks like. The one thing we are actually selling is that the numbers and claims on this site are honest. Spending that to convert a few more clicks would be the worst trade available to us.
If you ever find a fitment claim on this site that turns out to be wrong, tell us. We will correct it or downgrade its confidence level, and that is the single most useful message we can receive.