Thanks for replies. I agree 17 is not even close to reality. I do have some Mickey Thompson aggressive tires on also but keep them at 60 PSI. It is a big, tall, heavy truck but would like a hair better MPG. It was just at shop getting new water pump and tstat. I should have had him check o2 sensor. No check engine light ever on.

I was talking to an older guy with a Ford Super Duty V10 said he picked up 2-3 with muffler swap. I know apples to oranges.

Iowa is all blended crappy Ethanol gas, so that does not help. Unless u want to pay extra .30-50 a gallon.

Can I pull cat off or is there a o2 sensor after cat? not worried on emissions.

Not going to mess with hubs or full on exhaust.