I think the VDO Gas Control gauge works with the same principle. My Tito used to have one in all his rides before and he is able to get quite good FC as he can see which gear useage and pedal inputs gives optimal economy. This is were i first discovered that short shifting and lugging the engine (such as what taxi drivers do) is less efficient then driving steadily and shifting at the proper rpm range (gas flow is reduced as the car gets into its optimal power band).

Borrowed the photo from an online ad: