i have a 2007 Colorado, 2.9L, Z71, 4x4, 5 speed with 38650 miles. I get 15 in city and 19 on the highway. I had no idea my gas mileage would be like that. the factory window sticker says 18 and 23. I've changed the spark plugs, cleaned throttle body and MAP sensor. nothing improved. I have had a...