Variations in the amount of idling time as you sit and wait in traffic will have some affect on fuel mileage. Also, it seems that humidity, air pressure, air temps have significant effects on how this engine works, as does the types and amounts of other exhaust fumes your car sucks in.
ps. I've seen a test done in London, England years ago that compares the travel times of a car, a motor scooter, a bike and a pedestrian as they traverse the same course. The scooter was by far, the faster way to cross London. Next, came to guy on the bike, then the car with the walker coming in last. You guys sit in traffic for inordinate lengths of time. This kind of driving can produce dramatically different kinds of fuel economy from day to day.

 

I don't run into traffic due to my working hours, I'm not crazy enough do London anyway, it's a melting pot of crazies and tourists.

I take a 7 mile trip twice a day, car never stops apart from 1 to 2 sets of lights. I think it could be because I run a snorkel + a aftermarket filter, I think this setup degrades the filter real fast especially when picking up rain spray from the cars in front.

I checked the snorkel, filter and airbox today, the filter looks ok but it didn't help that it's dark gery in colour to start with. I was hoping for leaves in the airbox but it's clean.

 

Well I know from My experience that from dead empty (about a quart of fuel) that 5 gallons will net you about 2 bars under 1/2 full.

 

Try a new gas cap:


Low and inconsistent mpg

 

I suggest lubecontrol's fp60. www.lubecontrol.com