Ironically I have an old "fuel economy" gauge that runs on vacuum. I think it's for a 64 Pontiac Bonneville console, it's really a pretty thing. Anyway it goes from 24 to zero with a secondary color bar code under that goes from economy to power, economy being the 24 side and power being zero. So on a 64 big block Pontiac they figure the more vacuum you have the more economy you are running, based on the throttle being closed, and the less vacuum the more power you are making. It doesn't really consider that the motor is operating with better thermal efficiency with the throttle open because with the throttle open on a big block carbed Pontiac you will be going 100mph in a 1/4 of a minute or so and you can probably watch the gas gauge go down. In that case high vacuum, throttle closed as much as possible is where you get the best mpgs.
