Is it just me or are any of you getting a wide range in your gas mileage? I realize I ain't doing any scientific study, but my mileage varies between 19 and 25 mpg per tank full. I probably wouldn't think much of it, except I'm one of those guys that checks his mpg on each tankful. I'm trying to determine if any particular brand of gas will give me higher mpg. IS has 6700 miles on engine.
Tire pressure is very important, good call ISLover. Don't forget too that air temperature makes a nice difference (since colder air is more dense, more fuel is mixed with it). I'm sure that probably affects the VVT-i as well so it may be +/-'ing your timing depending on temperature which could amplify the gas used/unused.
I've noticed it too though I'm only on my second tank of gas.
Tire pressure and driving style are the mostly likely factors. Temperature not so much unless you think about it terms of AC vs no AC.
Yes at colder temperatures the air is more dense and the computer will inject more fuel per air volume, but it will be producing more power and thus for the same driving style, you should use less throttle. Net effect should be zero. Now on cold days you may spend more time letting the car warm up which will affect your mpg.