sounds about right, what size tires do you have?
my 2001 2500HD 6.0 4x4 4l80e 4.10s on 285 BFG mud terrains, custom pcm tune, k&n and exhaust,
i would average 15-16mpg driving around home (open country roads, averaging 55-60mph, mostly 30mi trips to and from town/gf house),
then 11 mpg when driving around school here in Raleigh,
13-14 on the interstate (averaging 75-80mph, would have done better if i drove a little slower, those 4.10s dont like the highway) and
about 10-12 pulling goosenecks
so your 12.5-13.5 sounds about right if you drive in town alot and highway
oh and first tank with the 37s and still 4.10s i got 12, and then 8 pulling my boat, this was not corrected for tire size, the other numbers were as my PCM was calibrated for the 285s