Gas mileage problem

You need to give more information. You could have any one of dozens of issues that could create that. It could be as simple as a dirty air filter to as complex as a brake dragging and creating additional friction. It could be as simple as under inflated tires? You could need a tune up or fuel filter replacement. Too many possibilities to narrow down, try to provide more information.

You may need to replace the fuel filter. A clogged fuel filter will decrease the gas mileage. Also if you are using a higher octane then recommended. Premium fuel "91 Octane" is the recommended fuel. The higher octane or even octane boost that you put in the car it will burn cleaner but it will burn faster, thus dropping in gas mileage

My 02 TL S-Type suddenly started getting 22-23mpg on the highway and we used to get at least 27 even up to 31mpg. I changed the air filter to a K&N, and a while after that we noticed the decline in mpg. There may be a little more performance in that it seems to accelerate faster, but I'm afraid the engine's computer adjusted for the extra air, and is adding extra fuel. Everything else is the same. Changed to Yokohama tires instead of the factory Michelins, but almost the same specs. Check tire pressure regularly... The only other thing I can think of is gunk/dust buildup in the intake because this car does have over 130k. Any thoughts?

Forgot, I thought these Acuras didn't have a fuel filter. (?)

Yes the 2002 Acura TL Type S has a fuel filter. Here is my source: http://www.overnightautoparts.com/storefront/?N=9293&Nr=OR(AND(make:Acura,model:TL,year:2002),AND(universal:1))&Vi=9496+4294963162+1562&y=2002&mk=Acura&md=TL

It's possible that the variable valve timing is stuck on the high performance/ power position (valves opening earlier and closing later). Check, clean, replace the variable valve timing solenoids.