Looking for better fuel milage

What size rear tires Tony? That does sound like low idle vacuum to me but I have not measured what my 340 has with a very wild cam to reference from. The light weight of these cars should give good milage unless a lot of unburned fuel is going out the tailpipes. My hotrod burns a lot of gas with that lumpy camand my heavy foot. Holley 850 DP carb with an aircleaner like yours.