All car computers know this even if they don't display it to you. The computer directly controls the flow in the injectors and the computer knows their flow rate (unless you swap them out, of couse).

First I saw was in a Saab 9000, around 1991.

Citroen C8/Peugeot 807 (same car, different badge). It's built into the trip computer.

My Subaru Liberty has one.

No offense intended, but this scares me.

They are ~everywhere~ for a long time around me. Quite a lot of driving instructors (specializing in 'reduced fuel consumption' trainings) use them as a general guideline to "see" how your behavior drives the consumption up.

Heck, mum and dad use that thing for years and drive even slower since they first saw that they could make their car use less gas.

You need to get a new car.. :)

My car is only 14 years old and still too reliable to need replacing, but I've seen a few newer cars with no fuel consumption gauge.