^That's exactly what we're doing. pre-ODBII vehicles are too diverse, every maker has their own interface system and they aren't the same so you can't make a device that works in them all. some maybe..definitely not all.
1991 Toyota Pickup 22R-E 2.4 I4/5 speed
1990 Toyota Cressida 7M-GE 3.0 I6/5-speed manual
mechanic, carpenter, stagehand, rigger, and know-it-all smartass
"You don't get to judge me for how I fix what you break"
P.S. I must be a wierdo as I think just because a guy can afford to do something, doesn't mean he should. I can afford to buy 100 gallons of gas several times a month, pour it on the ground, light it (or not)... but I don't think I should.
I use the odometer (with GPS correction because of my one-size too large tires) divided by fuel for my gaslogs. That is my real MPG benchmark because the ScanGauge is very reliable, but can get it's settings screwed up. The ScanGauge needs the odometer for adding tank-to-tank correction values, so the odometer is still the source.
For me, the ScanGauge is a real-time *relative* barometer, where I am competing against myself.
I've found the Scanguage to be more of a teaching tool than anything, and a 'gee-whiz immediate feedback system' second. Now that I have driven with it, I know what works and what does not, and for the most part, the same techniques are generally adaptable across most vehicles.
The only thing I really use it for now is to measure drafting efficiency. While I don't tailgate or even follow at a distance that's unsafe, it provides instantaneous feedback as to whether it's really worth holding that 2 second gap to the next vehicle or not. Aside from that, it's just entertainment.