How fitment checking works
Most parts sites answer one question: does this part fit your car? That misses the way builds actually go wrong — two parts that each fit the car perfectly, but not each other. SpeedSpec checks both, with written rules rather than a language model, so the same build gets the same answer every time.
A worked example
Here is a real check against the live catalog. The car is a 2022 BMW M3 Competition RWD, and both of these parts are listed as fitting it:
What the check returns
Put them in the same build and the cross-part check has something to say. This is the engine’s own output, unedited — the same list a published build shows, and the same one the configurator works from:
Reading the result
A conflict is a warning, not a lock. You can still add the part, save the build, and buy it. Plenty of people run coilovers and keep the springs on a shelf, or know something the catalog doesn’t. We would rather show you the problem than decide for you.
Warnings sit below conflicts because they are context, not contradictions — something to plan for, like an alignment after a ride-height change, or a spec we can’t confirm about your specific car from the factory data alone.
Every line above comes from a written rule with a written reason. Nothing here was generated on the fly, which is why two people checking the same build get the same answer, and why we can tell you exactly which rule fired.