This is an issue with old
EPA numbers. The EPA recently revised their mileage estimates for all cars, but haven't yet made those new numbers available in a format that sites like Fuelly can use. Here's the difference in numbers reported by the EPA for your model: 1998 GMC Sierra 1500 4WD
You can see that Fuelly has the old numbers. Until the EPA makes the new numbers available for download
, we're stuck. We could update a model here or there by hand, but setting the new numbers across thousands of models/trim levels over dozens of years isn't possible by hand.