Gas Mileage Computer & Cold Air Intake

I have a 5.3L 2004 Silverado Z71. I just installed the MAC Cold Air Induction system from http://www.rpmoutlet.com/newgmcsuv.htm

Now I don't know if you are suppose to follow that onboard computer or not but here is the scenario.

Keep in mind that I still have stock exhaust. All last week I would watch that instant average MPG meter. With the air on it would hover around 18-19 mpg on a flat stretch of road. With the air off, it would be around 20-23 on a flat road.

I put the Cold air kit on and I lost about 2 MPG according to that computer. What do you think of that? I did notice however that the numbers fluctuate really irradically now. They will jump from 18 to 13 and back. Before they would move more slowly.

Should I trust that computer? I am planning on putting on a free flowing muffler.

Now I have heard stories about the stock muffler being very very restrictive and some people say that it isn't that bad. Has any one actually seen the inside of a stock muffler? I have 3 inch pipe going into and out of the muffler so I figure If I just switch the muffler it will do pretty good. The pipe looks like it is mandrel bent. There are no wrinkles in any of the exhaust, that is another reason I thought of just getting a free flowing muffler and put it on.

Thanks

Andy

Well, my Grand Dad has a 2004 Silverado, and the instant MPG fluctuates radically, lol. On a flat stretch of road, going about 55 mph or so, it stays in the mid 20s to sometimes 30 mpg (jumping back and forth from 26-30). Up a hill, it falls to 6 or 8 mpg. Now the average MPG rarely ever changes (it may go up or down a tenth). The last time I drove it, it was averaging 16.4 mpg. The first tiem I drove it, it was only at 15.3.

The cold air intake should help your mileage, instead of hurting it, but I think your instant MPG computer is normal.

As for your muffler question, why not just go with a cat-back system, that comes with pipes. I know they cost about double of what a muffler will cost, but in order to do what you wnat to do, you'll ruin your factory pipes and muffler (in case you ever decided to swithc back to the stock unit).

but either way you decide, a high flow muffler will improve yoru mileage AND better yet, give you a much more aggressive sound (which could actually hurt your mileage, just becuase you will wnat to hear the pipes!).

I agree. A cold air intake should help your mileage a little if you keep your foot out of it! The K&N system on my truck sounds good by itself, but since I added a Gibson catback dual system, the whole thing sounds great!. The pipes themselves on the Silverados aren't all that bad, but if you really want to get the most out of a free flowing muffler, you should replace everything from the cat back. There's a ton of choices out there. Take the time to do a little research and get what you want. You won't be sorry.

Your mpg mode sounds like it's working like all the other trucks I've seen with it.

I will check the mileage through this tank of gas and see where it is compared to where it was with the stock system.