First the government website lists it as 19 city 23 highway 21 combined now but era rating on sticker when new I believe was 17 city and 26 highway. That said most would say with 50/50 driving about 22 mpg.
To increase mileage immediately for no cost and close EGR valve you could do ORM (off road modification) which is to unplug the mass air flow sensor this will most likely increase ,mileage about 2 mpg maybe a little more. Will only close ear if it is functioning correctly. This will also result in check engine light as long as is unplugged.
You also need to check to make sure the thermostat is operating correctly if when engine warm it should show almost straight up on gauge . Its common for it to show a little over a quarter if its not functioning correctly. There are several options to fix this if its a problem. Fastest and cheapest is to put Stant 192 degree thermostat inline in radiator hose. several other options and discussions on forum.
Also drive with overdrive on all the time not towing if you don't already.
Perhaps next time you get tires get slightly taller than stock up to about 30 inch diameter effectively gearing up about 5% depending on your current size. If you put tires so large that it requires a lift the mpg gain will be lost and may be worse.
For Highway mileage speed makes a large difference. Try going up to get into 5th gear at about 62mph, then back off to about 59 or 1750 or so rpm this should be the most efficient and should get about 28 plus mpg here any increase from that speed will decrease mileage in general
A tune from GDE will help a lot costs $500 assuming no other defects causing a problem.
They don't get great mileage if idling a lot.
Hope this gives you a start at least.
Thanks for all the great info. I have already done the ORM (just edited the original post to add that in).
I'll have to pay closer attention to the thermostat, I think if I recall it's on the left side, not fully up right.
I'll try driving with overdrive on.
My current tires are 235x70r16 (28.95 inch diameter) and factory is 225x75r16 (29.28 inch diameter) so if I went with 235x75r16 I'd be 29.88 inch diameter.
I also believe I have the F37 TCM mod since the shifting doesn't happen at 60 mph on the speedo and my RPM is 1900-2000. Although my manufacture date is 12-05 so that recall was only up to 11-05 from what I've read.