Id get a Vacuum Gauge for sure. It shows you how much of load ur putting on the engine when ur accelerating or cruising around. This is the one that i installed on my truck.
http://www.hallspeedshop.com/auto-ga...ge-p-2437.html
It shows you where the needle should be for best Fuel economy. I dont know if the Scanguage can show vacuum. Even if does, I doubt that it is as accurate and instantenous as an old fashioned needle gauge.
With the help of the vacuum gauge I went from 15mpg to 20.5 mpg.