I like at least 750 rpm with my foot on the brake in gear warm. Have had mine lower but it didnt help mileage any. You can set them lower but it seems like the crappy design of slant six intake manifolds just make them too "shakey" at idle in gear any slower and then surgy pulling away from a stop light. Also after you set your timing get a vaccum gauge and set your idle mixture screws for the most vaccum. Helps idle and part throttle cruising a bunch. I can pull 20 inches at idle with my stock super six when its all set right. And the best I have ever got mileage wise out of an automtic tranny sixer with a one barrel carb is 18 mpg. NEVER have I had one come close to the Super Six setup. Now that all 1970's stuff too. Never fooled with the 60's sixers.