European manufacturers implemented them into their factory gauge clusters, I fondly remember this old heavy Mercedes I drove once. Had a sweeping "MPG" gauge, was really useless as with those lead turds you had to floor it to get it to even move, so everytime you accelerated the gauge flat lined.
AutoMeter sells a digital version of those gauges these days, called an EcoMeter, but it requires a CANBus ECU, to which my ECU does not have (Ford started implementation around the 2002/2003 model year, but on carry-overs like the Taurus, it may not have gotten CAN until 2004 or 2005 MY). The EcoMeter is at least a little more calibrated to each particular engine than the old vacuum "MPG" meters.