This is an issue with
old vs.
new 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.
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