I've been carefully observing fuel consumption in my car (VW Passat 1.8T 5-speed automatic) for several years based on the MPG display on the dash. The following are best practices for my particular car:
- Speed up gently and slow down gently.
- Use the gentlest accelerator pressure necessary to keep you in the highest gear (5th = highest) as your safe speed permits.
- Use engine braking as much as possible (anticipate stops and minimize wasted energy from approaching too fast).
- Avoid unnecessary stops. They cost a lot of fuel. For this reason, avoiding peak rush hour can make a big difference to your overall fuel consumption.
- Speeds above 65 (in my car) significantly increase fuel consumption.
- Keep your tires inflated correctly.
- Don't use a roof-rack unless you actually need it.
Fuel consumption isn't linear because petrol engines are most efficient within a narrow band, and because lower speeds lead to lower gears which deliver less distance per rotation of the engine. Because of these factors, you need to learn your optimum speed to minimize fuel for a given scenario. If you have a regular commute, you can learn an optimum cruising speed as you approach a hill, for example, that will get you over the hill with a gentle slow-down and minimum down-shifting.
Air temperature also significantly affects efficiency. In New England, I get a variance of about +/-2mpg from winter to summer. 65-70 degrees seems to be optimal. Colder weather requires more time for the engine to warm up, which is the least efficient period. Hotter weather seems to reduce efficiency somewhat. A/C uses significant fuel.
My car has a Tiptronic transmission which gives me the option of manually shifting if I want to. Only very rarely do I find I can improve my MPG by manual shifting. The automatic carries a small (maybe 1MPG?) fuel "overhead" but because it adapts to more or less aggressive driving, it tends to do the right thing for good fuel economy if I drive driving sensibly.