Driving very calm(granpa style), the 4.3 (some 45,000 miles in it) is not reaching 12 mpg in heavy city traffic. But this figure isquite below the awaited mileage of 15. Can you point the 3 main issues that may be involved in the loss of mileage. Here climate is mild, but temp is always at the low side of the gauge (less than 1/4, mine is in ºC). Can a PCM check at the dealer indicate somethings (I doubt)?. Tires are 235/70 which I think is one size over. Tks.

 

A FULL Complete tune up will help you.
AC Delco Platinum sparkplugs, Cap, Rotor,Wires, PCV valve, Fuel filter, Air filter. Clean your MAF sensor, and your battery cables and post's. Also proper tire inflation is a must, Add good Fuel Injector cleaner every six months, and make sure your Front and Rear brake caliper slide pin's are clean and greased because they tend to sitck closed enough to kill your gas mileage, but not enough to notice anything is wrong.