fuel mileage drop, isx 15

Recently ive experienced a drop in my fuel economy, from 6.6-7.0 down to now getting 5.3-5.7. No noticeable power loss. Ive had a complete emissions delete done in Nov. 2014 {pdi} and went from 6.0 to around 7, then dropped to 6.5 during winter blend fuel season, then back up to 6.8 or so last summer. This winter the mileage has fell off badly. I did get new tires on both tractor and trailer, went to a non low rolling resistant tire all around but didn't think mileage would drop that significantly just from that. I only use Countrymark premium fuel delivered to my house. Will probably go in the shop this week and check the cac and do an injector test. Any other ideas?

 

Thanks. Am going in the shop on Wednesday I will have the tech look at those things as well as the injectors and fuel pressures

 

Cac is shot. Leaked 20lbs of air in less than 10 seconds and you could hear the leak from 10 ft away.

 

Well we're not suppose to discuss the delete but let me just say...mpg increased to 7 from 6, engine runs much cooler, fan hardly ever comes on, power is incredible, oil is cleaner {I can still read the word 'add' on the dipstick thru the oil after 15,000 miles} and I haven't seen the check engine light on since the delete. I should've done it sooner. Cac Is the charge air cooler