[bestclimb]

The HX has lean burn, the ultra and scan gauges will have a hard time with the varying air fuel ratios, which means it won't really know what your fuel mileage is. Ultra and scan gauges as I understand don't calculate fuel directly, they calculate fuel as a function of air flow and assume a constant fuel ratio. An MPguino is wired directly to the fuel injector signal and calculates fuel burn as a function of fuel injector duty cycle, which makes for a much more accurate reading when the car is capable of widely varying fuel mixtures.

a vacuum gauge is available at any parts store. but I don't find vacuum information very helpful once I have instant and average MPG info. The vacuum gauge taps in into the manifold and you can get an idea of the load the engine is under.

A scan gauge and ultra gauge plug into the OBDII port and know pretty much what the car knows from it's sensors. and can display a very diverse set of information.

The MPGuino is an open source project invented by one of the members here. It taps the speed sensor and fuel injector signal wires directly, and just calculates distance and fuel /hour and gets mpg from that. Less information is available but with an HX, VX or car older than 96 is the only viable option. I had an mpguino in my civic and I really liked it, on the road I don't find the increased amount of information the ultra gauge supplies to be that much more helpful for efficient driving.

I could be incorrect about the scan/ultra gauge's capability regarding direct sensing of the actual quantity of fuel. If so they make for an easy install.