Hemi w faint intermittant back fire

I have a 06 Ram 1500 5.7 Hemi with 80,000 miles. I bought used at 55,000, and the tach was flakey then. It would work right then not work at all, sometimes it reads more or less than it should. Just the other day it was working correctly? I really dont mind the tach, i'm wondering if it could be related to the real problem i want to address. I have an intermittant cough / faint backfire at idle, and generally during deceleration. I have changed the plugs air filter. plugs looked normal and even. I have no lights, or codes. I have seen mention to "cleaning the IAC". The backfire is very faint, and sounds like a muffled cough? When i bought the truck I belive the MPG was better too. I get 12.5 around town, and 15-17 Hwy. I can remember getting 15 / 19 before the back fire. Im not an ASE certified mechanic, but i have been turning wrenches, and doing most of my repairs for many years. I would welcome, and appreciate your input. Thanks in advance.
John

When was the last time some BG 44k treatment was ran through the gas? How did the TB look during the plug swap. You said all looked fair and about the same any look like they were running rich?

Any chance you could have gotten some water in your intake? (had this happen before and it was backfiring until I put in some additive to get the water out)

Also, check/test your fwd two oxygen sensors, as they let your engine know what air/fuel ratio to run based on readings from the exhaust and are directly proportional to your MPG's.