GMC SIERRA ENGINE POOR MILEAGE

Poor Gas Milage. 1997 Gmc 4x4. 350 Engene

I Have A 1500hd W/ 6.0l. What Would Cause My Mileage To Drop To 8.5-9 Mpg From 10.5-11? I Changed The Plugs, And Went To Synthetic Oil...

For The Last Few Weeks My Engine Runs At A Higher Than Normal Idle Speed When It's Cold. It Normally Runs At 1000 Rpm When Warming Up;...

Is Fifteen Or Lower Mpg Good Gas Mileage For A First Car?