Guides

A calculator answers the question you already knew how to ask. These are for the question underneath it — the arithmetic that behaves differently from how it looks, and the assumptions that change the answer without anyone mentioning them.

• How Much Car to Buy

  A car payment covers the loan and nothing else. Budget against it, and the rest of the cost arrives anyway.

  6 minute read

• What “Plus Tax” Adds

  There is no single answer, and the honest version is a range — but it is a narrow one, and you can work out your own in about ten seconds.

  5 minute read

• Stacked Discounts

  Two discounts never add. And the amount by which they fail to add is something you can work out in your head.

  4 minute read

• Markup vs Margin

  They describe the same money and they are never the same number. Confusing them costs you on every sale, quietly.

  5 minute read

• The MPG Illusion

  Two cars. The one that improves less on paper saves more than three times as much fuel.

  5 minute read