poor fuel mileage

What type of plugs did you use? Anytime I put any other plug than AC Delco in a GM engine, the customer comes back, That is the only plug I recommend for GM engines. and it is always the customer who says to put the other plugs in. That could be one of your issues. A other thing is that cars lose some MPG as the cars get older and or more mileage on it. And what plug gap did the mechanic use? Alot just take the plug out of the pkg and put it in the car... If you still have issues, ask the mechanic to hook up his computer to your OBD port and take the car for a ride. His "scanner" will be able to see what you sensors are reading out while the car is driving. Best of luck.

Thanks for the response. I used Bosch platinum plugs, but the fuel mileage is exactly the same as the old plugs(and the car runs beautifully, as it did when it was new). I have no idea of what the spark plug gap was set at, what should it be? I own a 2001 Silverado with a 4.3 liter and 75,000 miles on it(it still has the original spark plugs), and it's fuel mileage actually improved as I put the miles on, not the other way around. I will ask the mechanic about the scanner, good idea, thank you.

change your ignition coil on these cars if you see a dramatic drop in mpg, along with wires and copper spark plugs. fyi always run full synthetic oil not blend full synthetic Castrol oil.

I have a 2000 Chevy Cavalier I lost my MPG on change fuel filter plug plug wires and everything you can think of and I'll still lost my MPG