Okay, so from what I understand engine vacuum is basically your engine load and keeping a higher vacuum means less engine load and better fuel economy. I recently installed a vacuum gauge and now my question that my friend are debating about is that, at say 65 MPH in overdrive, my RPM are around 1600 and I have 5-6 inches of vacuum and if I'm in 3rd gear (I have an auto so only 4 gears) my RPMs are at 2100 and I have about 10-11 inches of vacuum. (About 13-14 inches at idle but let's keep the debate on which way would get better fuel economy and not anything specific about how my engine should be at a higher vacuum or RPM or anything like that)

Thank you!

 

http://www.ehow.com/how-does_4672496...y-driving.html

 

Thank you but I was more asking about which gear based on my info would net better fuel economy at a constant highway speed not which speed would be best. Any input on that?

 

At 65, 4th.