I do mostly city driving, and my trip auto ranges from 10mpg-12mpg (Although I drive 2 miles to work), and my average mpg on the last tank was 13 mpg. It seemed to be getting around 15-16 when I first got the car and was doing 26mpg on the highway...

I'm using Chevron Techron.

My AC is generally blasting because it's hot here.

I wasn't sure whether to bring it in to get it checked out... Seems really odd.

 

A/C can make a significant difference if you live in a hot climate. The question though is whether anything else has changed about your driving behavior? More roadworks? Change of route? etc.

 

I was driving a tad more aggressively, but didn't think it would make a difference..

How accurate is the estimated fuel economy for short journeys?

 

Hi bluesman78,

While I have an S-Type, what I've noticed is that for short journeys only gas mileage sucks. It takes about 30 or so highway miles to get the overall mileage up. Another factor is that if you have some traffic snarls in your commute. One or two of those a week will drag your mileage down quite a bit. Living in the DC area I'm painfully aware of that. I would recommend perhaps three week's worth of data.

 

There is a huge difference between stop and go fuel economy and highway fuel economy with this car. Mostly this is due to the tall gearing of the transmission in 6th gear and partly the excellent aerodynamics of the body.

You can expect consumption on the highway to approach 60% of consumption in City driving.

Aircon will make mileage even worse but will have less effect at highway speeds than in stop and go because, ironically, the tiny additional throttle opening required to cope with the AC compressor load actually makes the engine a tiny bit more efficient at steady throttle openings.