Nissan Repair: 1995 Nissan Pickup- Transmission, Gas Mileage, nissan pickup, gas mileage

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
Hi Ashley -

It appears that it might be the transmission, however there might be a few things you can do before the costly overhaul. Have you changed the transmission oil and filter yet? Make sure you flush out the torque converter with fresh oil, too. Also, Carquest sells this wonderful trans additive AT205 that works wonders for slipping transmissions.

Also check the timing belt. The clanking, or pinging, might be caused by the belt slipping a notch and the car is running too advanced. That would rob gas mileage as well. If you checked the timing during the tune up, then it is probably OK. Are you running high enough octane fuel?

Hope this helps.
calvin