Diagnose

A calculator answers a question you already know how to ask. These are for the other kind — one question at a time, ending at the thing to check and the tool that does the sums. Every flow works without JavaScript; the whole thing is on the page.

// Flows

Will these wheels fit?

Fitment fails in four places, and only one of them is the bolt pattern everybody checks first. Work through these and you will know which number is about to bite you.

Why is my AFR off?

A mixture that reads wrong is either genuinely wrong or being reported wrong, and the two need opposite fixes. Start with which way it is off.

Where is my quarter-mile time going?

Estimate your ET from power and weight first, then bring your time slip here. The gap between them tells you exactly which part of the run is costing you.

Why a flow and not an article

Most tuning problems are not hard because the answer is obscure. They are hard because there are six plausible answers and the order you check them in decides whether you spend an afternoon or a fortnight. An article lists all six; a flow asks you the one question that eliminates four of them.

Each of these branches only on things you can actually observe — a gauge reading, a time slip, a measurement off the car. Nothing asks whether a part is faulty, because that is the thing you are trying to find out.