not 100% sure about the 2nd gen trucks but on the 3rd gen it is referred to as the "Lie-O-Meter"... calculate your mileage by hand and see how it compares. also reset the overhead... I had one drop 5 mpg after leaving it alone for an extended period of time. reset it and see what it says.

 

it takes your computer a bit to adjust to ur driving when i got my truck it said 8 mpg cause the previous owner hammered it hard. i got it up to like 11.3 now same set up as you, 35s, lift, exhast, round air filter.... i do ALLLLL city driving i hit the highway maybe 20 miles a month if that. im average of about 11-11.9 never hit 12 lol, i drive agressivly at times and like an old man at others so your prob about what you should be getting is fair ide say

 

I just recently had dual exhaust put on my truck at a very reputable auto/exhaust repair shop. I told him to put on a high flow cat. He quickly told me that he did not want to sell me one stating that he did not think it was worth it. He claims to have cut one open and compared it to a standard cat. After inspecting the cats he says that he could not find any difference between the two. Why would he lie his way out of a quick buck if you all disagree?

 

the high flow cat has sheet metal inside to smooth the exhaust flow out of the cat. I had a shop show me the difference, on both ends, if you put your fingers in a regular one vs a high flow one you'll see what I mean.

 

The high flow might have the same size honeycomb structure but if it is physically larger or like Steve said it is smoother flowing inside it should flow more gas and be less restrictive. At first glance it may not look like anything special

 

I see you are in Maine. As it get colder your mileage should and will drop some.

I reset my mileage counter whenever I fill up. It's dead on everytime I check it.

I too have been through the hi-flo cat debate a year + ago, and I also don't believe the hype. Cats have said to only have a 2-3% back pressure rate.