l drive an all stock 1997 gmc sonoma the truck has the vortec 4.3 v6, and an auto tranny, until about two weeks ago my truck was getting 18-20 mpg and now its only getting 13-14 it has had a tune-up done a month ago too. Does anybody have any ideas on what l can do to fix my mpg??

Does it run good?? Need more info!!!

Scan the computer for faults. Is the service or check engine light on? There are lots of computer controls than can fail and cause poor performance and mileage. But you wo not know until you scan the engine computer.

Do you have a locking gas cap ? If you have a air leak between the MAF sensor and the throttle body that will do it or a bad o2 sensor will also do it.

Obviously something changed. The 4.3L Vortec has had it is problems with the fuel injection system. GM has extended the warranty on the injectors to 200,000 & r replacing the original pop-off type injector nozzles with solenoid type injectors. If ur vehicle has more than 200K the kit costs about the same as the original ''spider'' assembly they had in the past. A leaky injector nozzle or fuel pressure regulator can cause a drop in fuel economy. First l would address the sudden drop in fuel economy with the guys that did the ''tune-up'' BTW the term tune-up came from generations of vehicles that required periodic adjustments. Example: ignition points needed to be replaced every 12K miles because of mechanical wear. Starting in 1968 GM introduced Electronic CD ignition in the Corvette, After 1975 almost all cars & light trucks had electronic ignition & adjustments were not really required. Carburetors also started to disappear in 1975 Datsun/Nissan Z cars, VW (1968)and Cadillac started using electronic fuel injection. No more carburetors to adjust. Hydraulic valve lifters were standard by 1957 in most US cars, so no valve adjustment (in theory) was needed. To make this long post shorter l will just end in saying since around 1985 most cars & trucks do not get ''tune-ups'' just maintenance service such as filters & spark plugs. Ignition wires get replaced when they get broken.

Have u also noticed a change in the performance?

BTW, the 200K warranty may only apply to Calif. vehicles but if the injectors r the problem it wo not hurt to ask, the worst they can say is no.