my dad (and now me) usually drive UNDER the constructor's annouced millage: for my dad's 10y old Saxo, the mileage is (sorry, but youll have to convert) 5.5l/100kms.
My dad manages 4.7/4.9/100kms. This in-city driving (but not stop-n-go driving, just regular, empty roads city driving). This is taken from the last 100THOUSAND kilometers, so should be pretty accurate.
I do with my Xsara picasso (also a diesel) 6.0l-6.5l/100 - constructor says its 6.5l/100. But I got more of a lead foot than my dad.
How? (keep in mind these are diesels).
High tyre pressure (also helps decrease tyre wear)
Mid-75% throttle on accelerating (like out of semaphores).
Keep the revs in the mid-range (in the saxo, around 2.5-4.5k (goes from 1k to 7k), the Xsara doesnt HAVE a rev-meter).
Get to top gear, and COAST. Putting it in neutral uses MORE fuel than just letting it coast in gear. Thats cause when in neutral, it needs a dab of throttle to keep the engine going. When coasting in 5th, the wheels are pushing the engine, so it uses _no_ fuel AT ALL.
Thing is, when coasting in gear you will be decelerating, so if you want to go long with a near-constant speed, THEN use neutral. usually, though, you will want to keep coasting.
Never ever ever EVER brake. Coming up to a red light? Keep shifting down and let the good'ol engine brake for you. All the way down to 1st (where you will be going @ 5kph), then neutral and just brake when you'r at 1kph.
- think of it this way, since the engine is braking for you, it doesnt use any fuel. When you consider you will have to brake nearly 500m BEFORE the red light, think that you will be doing those 500m FOR FREE.
Also, if you got a diesel, consider filtering and re-using your frying oil
I know for a fact it works on just used frying oil, I just dunno if it would work with a diesel/oil mixture.