I have a Vacuum gauge in the Bitch, great add on, I watch that one the most to be honest, get better mileage watch for other issues.... Get rid of that extra volt gauge nobody watches and install a vacuum gauge, you'll be glad you did.
I've got a Vac Gauge (Perf. Indicator) in my '74 Duster from a '69 Cuda and a Tach. It is most definately one of the most watched gauges in my car (that and the fuel gauge
) Of course that Vacuum Gauge being right in the Center of the Rallye Cluster it's not hard to notice it
And the Tach is mounted in front of the A/C Vents (not my most brilliant idea..blocks some of my cold air) so i notice it when i slap gears, and i can still see it quite well while driving.
My Duster gets it's best mileage at 50 - 70mph, pulling around 20" of Vac and 2,000 - 2,700 RPM. You get the RPMs and the Vacuum both lined up in their sweet spot and a vehicle will net decent mileage.
Now at an idle my Duster only brings in about 12" of Vac (484 Purple Cam) but once the rpms pick up it'll gain vacuum obviously.