Tachometer needle jumps high before stepping on accelerator pedal

Truck was running fine until I replaced a new flow master exhaust muffler a month ago. Everything was good until I started noticing low mileage on my gas. Check my intake filter and was not dirty, took out the MAF and throttle body, check not that dirty but gave it a clean with appropriate approve cleaners. Reset the vehicle ECM for memory flush out of old behavior to relearn new setting. Did the throttle body relearn procedure three minutes idle run, sixty seconds engine off, another three minutes idle run and idling was between half of 500 rpm and 1000 rpm. Now comes the trouble, when vehicle is put in drive the rpm needle suddenly jumps high before I get to even step on acceleration pedal and that never happened on old ECM brain. Drove the truck above forty five miles with lots of deceleration as said for throttle relearn procedure and nothing has changed. So, what am I missing and what should I be checking? Any feed back will highly be appreciated. Thanks