The volume of air required to operate the engine is not determined by speed, it's RPM. So unless you're driving the entire time in the same gear, I don't think that's your problem. Now a ram air intake will force air into the engine based on speed but I don't think yours is a ram air system.

Also the MAP sensor in our trucks (MAS sensor in other vehicles) measures the pressure/volume of air entering into the engine. The PCM/ECM will add the appropriate amount of fuel to meet the predetermined air/fuel ratio that's needed and confirmed by the O2 sensors after combustion.

All in all, there are a number of factors that could be contributing to the decrease in fuel mileage but so far you believe it's the intake. My suggestion would be to replace it with the factory one and see if it improves.

 

Installed mufflex 24, should economy improve?

 

I saw no difference in stock muffler and a 24 inch Magnaflow.

 

My understanding behind the idea of having a better filter, CAI, or ram air intake comboed with a new muffler is it allows the engine to run more efficiently. Allows more air and cooler air to enter the engine and the the change in design of muffler allows for less back pressure in turn allowing more exhaust to leave the vehicle. An engine that runs more efficiently in turn should get better mileage. I'm no expert but that was my understanding. I like the sound of my 18" Mufflex and the look of the Airaid CAI, but I never expected for mileage to drop. It could possible be other factors I'm not aware of, I'm new to the world of engine mods.

I could understand it staying the same but not dropping. The drop occured on my in city driving but my highway mileage went up. It confuses me but then again I'm only on my second tank since adding the mods and switching back over to 89. I am getting a little better mileage now though after reseting the computer (well I think I did), back up to around 16.5 MPG. We'll see how it goes after this tank.

 

i have notice my MPG drop too. whn i first got my truck i was in the 19 mpg range and than with lift and tires (3 months after i bought thte truck) i went down to around 15mpg. i than added a intake and what not and i was seeing aaround 16.5 but now im getting more around 14.5 i drive 1200Kms to work and its the same route everytime so why does the milage keep gettin worse?

 

I'm learning if I really pay attention to how I drive I can get it back up near 18 MPG if I can get the MDS to kick in most of the time. I'm curious to see if I went back to 87 if I'd get better mileage. We'll see, I do like the sound...except when the MDS kicks in and it has that weird frequency. Averaging about 16.3 MPG according to the computer, only 2nd tank though and I've been a tad bit squirrely with it as of lately.

 

Well, KM=Canada, which means cool weather. That is your issue. It will pick back up in Spring.

Todays cars run richer when the air temp is cooler.