Which fast-food item gives that informal currency index its name?

The Big Mac Index works because a Big Mac is made almost identically in dozens of countries, so its local price reflects local costs of labor, rent, and ingredients in a fairly consistent way. Converting each country's price into a common currency and comparing it to the US price gives economists a rough gauge of purchasing power: if a Big Mac costs far less than expected once converted, that currency may be undervalued.

Coffee and bread would seem plausible too, but neither is made to a near-identical global recipe, since coffee culture and bread styles vary enormously by region, making cross-country comparisons far less reliable.

The index spawned playful cousins, including a "KFC Index" for parts of Africa where McDonald's has little presence, showing how a gimmick can still carry a genuinely useful economic idea.