I used to write down my fuel efficiency on a tank-by-tank basis. Overkill. Now, I just watch the number in general.

To me, the biggest advantage in watching your fuel efficiency is that I take it as a GENERAL indication of the performance of many (not all) components of your car. While I am very big on preventive maintenance, I'm also big on monitoring - visual inspection when possible, listening for sounds when driving, smelling things (inside passenger compartment, engine compartment, fluids, ets.), and the old TRIP/RESET MPG indicator. If I happen to notice a dip in the MPG then it signals me to look into it.

There are so many things on a car that can contribute to your fuel efficiency, I'll just name a few:

1) Inaccurate/broken gauge.
2) Carrying objects on top of the vehicle.
3) Driver misunderstanding the reading on the gauge.
4) Weather (i.e. wind & direction).

Most times that I drive I don't care about the MPG. I want to get where I need to go, as soon as possible. When can I speed up? When should I slow down? What's my current ETA? I've got many other things to think about. But sometimes when I'm not in a rush to my destination, I try to keep a steady 60mph to check my MPG.

I am very curious to hear what kind of MPG others are getting on their CLK.

I have a 2006 CLK 500 with 43k miles. The few times that I've 'attempted' to keep a steady 60mph on a 200 mile non-stop trip the highest MPG I got was about 28.3.

Yesterday, I took another 200 mile highway, non-stop drive and again set my cruise control at 60. I was shocked to see that the MPG hit 30.0. Is this a fluke? The only changes I made since last checking my MPG long on a 60mgp long drive were: brakes, brake fluid, ps fluid, differential fluid, and new tires. The only maintenance item that I would think COULD be the cause is the new tires (new Michelins vs lousy Pirellis). But I'm thinking it might've been caused by the weather (perhaps wind at my back).

Your thoughts?