I get around 15-16 in the city, 17+ on the interstate. Recently got 20.5mph (record high) returning from flagstaff (downhill for 20 miles, plus speed was 70mph). Normally at 70mph, I can get around 17.5-18 in AZ (chevron 91). In So. CA, I always get 2mph LESS than in AZ, carrying the exact same loads and running chevron 91. My hunch is that the 91 oct gas in CA has more ethanol to boost the octane. Since ethanol has less energy than gas, you get fewer mpgs. Also, there are zillions of traffic lights in Orange County, one practically every 1/4-1/2 mile, and lots of steep hills. Here in Tucson, the hills are gentle, air is thinner and warmer.
I also learned that when it's cold, your mileage will suffer. During the summer in Flagstaff, I can get 17-18mpg driving around Flagstaff (7000 ft) and on hwy 80, but during the winter, the same drive only gets 16mpg. My guess is the denser/colder the air is, the richer the mixture (eg, more air, more fuel), and hence the more fuel consumption. In fact, I get my best fuel economy in all my cars during the summer when it's 100F outside. The air is thin, engine doesn't require any warming up.
You could try running a higher tire pressure than the recommended 29/32psi. I found that that pressure was too low for my LX (unloaded), as my previous tires (Bridgestone dueler HT's) were worn more on the outer and inner edges rather than the center, indicating underinflation. I had them at 32-33 psi, all 4 tires. With my new Michelin LTX/MS2's inflated to 38psi, I am getting about 0.5mpg better than with the Bridgestones (probably because of the higher pressure) and the ride quality is similar (not any harsher due to the higher pressure). Anyways, I'd try a higher tire pressure like 36psi.
My mileage (or is that kilometerage) tips:
1. Use high octane petrol 98 rating with extra octane booster
2. Inflate tyres to 38psi with nitrogen
3. Remove all excess weight - third row seats and spare tyre
4. Drive at constant speeds and avoid hard breaking/acceleration (maybe watch the "econo-guage" in the info display)
5. Slipstream trucks
6. Coat my paint with Teflon spray
7. Fold side mirrors in flat
8. Keep the radio antenna down (listen to mp3's)
9. Keep the aircon off
10. Don’t turn on the lights
11. Don’t use the fridge in the centre console
12. Use a grappling hook to get a tow when I can
13. Remove the air filter
14. Disconnect 4 spark plug leads (alternately around cylinders)
That's all I have thought of doing - any more suggestions?