Nissan Repair: 1995 Automatic Nissan Pick up, lockup torque converter, fuel pressure regulator

Question
I have taken my automatic pickup on a long trip. On the way home (after about a month of being relatively unused) it began to get dramatically less miles/gallon-- from ~17 to ~8 mpg.  Frustrated, I checked and replaced oil, spark plugs. It still got 8 mpg. It has also began to shake/vibrate while in drive or reverse, but not while idling. It makes a clanking noise, somewhat like metal on metal. It has about 147,000 total, and received a tune up with oil changed, coolant, etc prior to trip. A mechanic has suggested replacing transmission.  Is this necessary? What can be done?

Answer
Ashley,

If the transmission is dragging internally or the lockup torque converter is not working or the transmission is stuck in limp mode can cause the poor mileage.  But, before I start looking at the transmission I would look at simple things like a new fuel filter and to make sure the fuel pressure regulator is working and you have no leaky injectors.  If all of this is fine then look at the transmission.  The metal om metal noise could be as simple as an exhaust hanger being broke and the exhaust hitting on something under the truck or a broken motor mount.  Both of these are cheap repairs.