I average 13.5-14 in combination driving with 4" of lift and 35" tires. I'm currently on a trip "home" and averaged 16.3 getting here (360 mile trip, about 300 interstate) so I'd say you could probably use a tune-up.

 

My truck is at 56k miles now and gets no better than 10 in town and around 14-15 on the highway. I admit I don't drive like a saint -- I use the power when necessary (or fun!) [8D]

I know it's overdue for plugs & wires. Maybe that'll net another mpg or two more.