Generally, expect figures in the range of:
* 14-17 mpg city / 17-20 mpg highway for models with larger V8 engines (most common). Lower end of the range is more likely for 4WD models or heavier towing.
*Slightly better mileage might be achieved with a smaller V8 (if one existed in that year), but those were less common.
You'll find the most accurate information on the specific fuel economy of a particular 1999 GMC Suburban in its owner's manual or by looking up the EPA fuel economy estimates for that specific vehicle's engine and drivetrain configuration online (using resources like fueleconomy.gov, though data for older vehicles might be limited).
