gas mileage decrease

[m2628jp]

I have an 01 Tahoe, 5.3, K&N drop in, magnaflow muffler. My gas mileage has recently decreased noticeably. For example, I used to average about 15 mpg around town, and 17-19 highway. I took a 320 mile round trip last week, my usual 75mph, and got 15mpg with all highway driving, no idle time or traffic jams! Around town it is about 12! I have also cleaned the MAF, with no change.

 

Any ideas would be greatly appreciated. I'm saving $$ for a Westers PCM, but this gas mileage is making it difficult!

[apache sierra]

have you changed anything like tires or gas? I had a 99 jimmy with 4.3 and i went from non oxygenated fuel to oxygenated and noticed a decrease in milage of around 4mpg . Then i changed gas companies and noticed my milage went up about 2mpg. Itried a tune up and lighter weight oil but didn't help much.

[tmf6902]

maybe time for a spark plug change??? Just a guess

[m2628jp]

Forgot to mention, I replaced the plugs with NKG Iridium and new wires about 10,000 miles ago. I am also running non-oxygenated fuel.

[citydriver]

If you put bigger tires on it,that would affect the math if you have'nt had the speedo recalibrated...not to mention add weight to the trk....are the NGK's gapped right? that would be somthin else to chk