I read an article in my 4wd mag where thay tested a performance throttle body. In the end it said since the vacumn had dropped they knew more air was getting into the engine. They also noted a power increae of about 5hp. I have vacumn gauges in my truck and my mustang. The one in my stang actually has markers on it indicating Bad, Good, better, Deceleration for MPG. But the way it reads, the more vac. you pull, the better mileage you are getting. Is that how it works, less vac/more power and more vac./better MPG? I just installed a CAI on my truck and noticed it caused my vac gauge to drop a notch. Just trying to figure out exactly what I have done. Feel free to correct anything I said, as I'm trying to learn a little here.
That is pretty much it, but you really need to look at the two different cases.
Case 1 - Cruising down the highway at part throttle. This is where you are interested in MPG and where the higher vacuum number gives better MPG*. The vacuum is just a way to measure the opening of the throttle plates so basically, keeping the throttle light and even gives you the best MPG.
I put the "*" in and the note here is "all else being equal" If you have a dirty air filter or a restriction in the intake that makes you open the throttle more, you may see higher vacuum, but because the throttle is open more, you are dumping in more gas which could hurt MPG.
Case 2 - Wide open acceleration. In this instance, you are concerned with power. All the air that the engine uses has to fit through the air filter and throttle body. Depending on which element is more restrictive, that is what governs the vacuum at WOT. If the most restrictive part does happen to be the air filter and you replace it, you may see the vacuum at WOT decrease slightly which should indicate more power - again, all else being equal.
But, when you go back to highway cruising, the throttle body (and mostly closed throttle plates) is a far greater restriction than the air filter, so your vacuum should be back to normal and along with that, the mileages should be back to normal.
The last thing to consider is the CAI itself. If it really does it's job and picks up colder air, this could cause a little drop in MPG simply because a little more of your gas is now going into just heating the air and because cold air is more dense than hot, it requires more fuel.
On the up side, the difference will probably be very small. At highway speeds, you could literally have hurricane force winds blowing under the hood, so the air temp tends to be pretty equal no matter where you pull it from. Around town, you might see a little gain by pulling air through a CAI.