I have a 02 gmc 2500 hd with about 31,000 miles. I was getting around 19-21 mpg. hwy.but now appears to have dropped considerabily, 15-16, nothing has changed, no oil in crankcase,no lack of power,all filters are good. has anyone heard of bad fuel problems ? or have any ideas were i might look. thanks .. retiredalaskan in florida.