I finally managed to install a vac gauge around new years. It wasn't too hard except for a square bit plug that wouldn't budge despite lots of WD-40 treatments. To make things harder it was impossible to find the metric sized driver (7mm?); after mangling it completely with my wrench/plier attempts, I filed it down to the english size (1/4?), and got lucky with the drive and an 18 inch iron pipe extension for extra torque. The rest was easy. I still have to find a permanent spot on the dash for it, where I don't take my eyes off the road,
Driving with it, it surprised me on several things. Most notably, the fact that idle operation takes so much fuel per revolution of the cylinder. At idle, the needle displays -21 or -22, and at deceleration fuel cutoff, it's closer to a vacuum, around -25. I believe true vacuum would be around -30 pretty much. The units of press are inches of mercury or something.
So, assuming the air to gas mass ratio is constant (14), you can easily get the amount of fuel by simple arithmetic. The amt of fuel per rev of the engine is simply proportional to the vacuum reading (as a neg nr) plus 30, due to the only equation i remember from thermodynamics, namely, that pressure is proportional to density (rho) times temperature (T, in kelvin):
p = rho R T or maybe it's k, boltzman's const, instead of R, i forget.
Anyways, idle, at -22, takes 8 units of fuel (30-22). I get very decent light power at a reading of 15, which is 15 (30-15) units of fuel, which is less than just twice the idle amount of fuel, which is great for maintaining speed at lower speeds like 30 mph.
So, idling at 800 rpm takes almost a quarter the fuel it would to get decent light power at 1600 rpm with a fuel consumption of 15 units.
At 20 units of fuel (vac reading of 10) I get pretty awesome power output, and I'm likely running near optimum efficiency from what I've gathered from BSFC images I've googled; this is about 2/3 rd wide open throttle. Anyways, I think I want to stay at around that reading of -10 within plus or minus 5 to be in the optimal range.
At -5 I'm using 25 units of fuel, or 83% wot. I try not to go past that, as you start loosing a bit of efficiency as you proceed toward WOT from there.
As for mounting the gauge you could turn the gauge 180* in the metal holder and find a place to mount it underneath the dash. I've had a similar set up using a 3 pod gauge set (oil pressure, amps, water temperature) before and could glance at all three gauges in a matter of 1-2 seconds. I made a bracket out of a 90* cabinet corner angle for my vacuum gauge and was able to secure it to one of the plastic pieces of trim with a single screw at about the same elevation as the top of my steering wheel in my '88 Escort. Depending on the age and condition of your car whether this is an option for you or not and whether your gauge could be fastened to the angle. If it's an option you can buy these angle brackets at about an hardware or home improvement store in various lengths from about 1" to 6".
HC: I saw your DIY fuel rate meter link; pretty creative, and nifty, looks like a lot of tinkering too. I suppose they both can tell you the same information if you are very fast at multiplication and division. (If you're not as fast as a computer at this artihmetic) I think the amount of fuel (or air) per cycle of the engine is a more meaningful reading than the burn rate for hypermiling purposes (figuring out when you're running the engine at very inefficient states, and taking corrective action).
Fordman: thanks. I'm going to get to finding the gauge a permanent spot by the end of the month or so. I glance at the gauge quite a bit, and even though it's somewhat central, it's below the horizon of the hood by a bit; I feel like I'm slightly sacrificing safety with eyes straying often off the road, even if just for a second at a time. I want to avoid mounting locations that are too far down. With how often I'm looking at the gauge, I'm going to mount it on the dash so I won't have to glance down and lose sight of the road.