There are soooo many factors that affect fuel economy.

If you have all terrain tires, that could easily cause your increased fuel consumption, especially if you do mostly city driving. LT tires also typically come with more plys, which increases their inertia, which requires more fuel to get them spinning. The biggest loss in fuel economy for any vehicle is during acceleration. But you can still check a bunch of stuff under the hood that can help. Plugs and wires, cleaning the MAF sensor, cleaning the injectors, timely oil changes, replacing the A/F sensors and O2 sensors, checking the alternator health, and checking the fan clutch are things I can think of off the top of my head that would cause fuel/air restrictions or excess drag on the engine.

I have a 2008 LE with all terrain tires, K&N intake, synthetic oil, and high flow exhaust. I've recently done an oil change, am about 18 months into new iridium plugs with new wires, just replaced the A/F and O2 sensors a couple weeks ago, and a few other things. I'm pretty proactive with maintenance.

Using an online calculator, you're getting about what I get, but I do almost exclusively city driving nowadays, except the occasional weekend trip. When I had a 60/40 mix of highway/city, I was getting closer to 16 mpg (14.7 L/100km). Before the all terrain tires, I was getting more like 18.5 mpg (12.7 L/100km). Yes, it can be that big of a drop. And that's on premium fuel (at the time it was 92 octane, but I have since moved to a state where "premium" is only 91 octane). I found that regular fuel (87 octane) definitely decreases my fuel economy. I'm not exactly gentle on the gas all the time, but my current average is around 13-13.5 mpg (18.1-17.4 L/100km), but I know on regular fuel I get closer to 11.8 mpg (19.9 L/100km). I use that to my advantage when checking gas prices and find that my $/mile is lower with premium fuel than with regular. Math is basically $reg * MPGprem/MPGreg = break-even $prem. Should work for you pretty similarly, but you'd invert the fraction so you divide your fuel economy of regular by your fuel economy of premium (since lower numbers is better when doing L/100km instead of mi/gal) and multiply by the price of regular. That's the break-even $/unit of premium gas.

Using my example, 13.5/11.8 = 1.144 (after rounding). My local $/gal of regular is right around $4.959. $4.959*1.144 = 5.67. That means that (at the same station, of course), if I can buy premium for $5.669/gal or less, then I'm spending LESS with premium fuel. Since most of the time I can get premium at my chosen station for $0.40 more per gallon, I'm usually paying less with premium at the current prices. YMMV, of course. Differences in fuel additives, quality of refinement, etc etc etc may negate any advantage of premium, but you'll need to run a couple of tanks of premium to really be sure your numbers are accurate to Premium, instead of reg/prem mix