I just took a long trip in my Blazer(1000 miles). I changed the spark plugs before the trip home. I tracked the fuel economy on the way home and it was about 17-18 on the highway. I used to get 22-23 highway. I had noticed a little smoke recently coming from the exaust upon start-up. This was even before the spark plug change. Is it maybe running too rich? What can I do at this point? I have about 97000 miles on the vehicle.