I've only had a couple of cars with fuel consumption on the dash, acuracy is near enough, and something to keep you entertained. With my Volvo nothing at all made a difference to fuel economy except the right foot. Oils from 5W-30 to 15W-60, AC, weather, tyre pressures, showed no difference in fuel economy. On a quiet night on the motorway with no traffic it would show some impressive figures, towing a loaded trailer would use twice the fuel...get it on boost and it would suck fuel.
Now I have a Honda Airwave (Honda Fit stationwagon) and everything effects fuel economy - AC, tyre pressures, weather, it all shows on the gauge. I've just topped up the oil with 500ml of 15W-40, and fuel economy has dropped considerably. The design focus of this car is all about fuel economy, and it shows.
I'm fighting it, but I might have to convert to the Thin Side...