 Well, here's the thing. With a 98 Tahoe, if they're famous for having fuel mileage issues, if the converter's been removed and the intake's been changed because it changes airflow, legally you're supposed to have all that stuff on there, blah, blah, blah. Disclaimer. Anyways, find a custom tuner that tunes these vehicles. They'll be able to tune it, and it should bring back some of your fuel economy and also give you more power, too, because it's going to be a tuning issue that's going on because the airflow's been altered majorly from factory with that air intake and the exhaust.