That's impressive. It definitely helps doing some slipstreaming. At this time of year it makes a right mess of your car though with all the rubbish getting thrown up!
Yeah, it does make quite a noticeable difference! Although I've noticed a few lorry drivers don't like you doing it and keep moving over halfway into the hard shoulder or slowing down to try and make you overtake them!! - Definitely agree with the dirty car comment! Almost pointless cleaning it at the moment anyway!
A good tip is to lift off going down hills I find. If you check the instant mpg it goes to --.- and no petrol used at all. Lifting off a bit before junctions and roundabouts helps too.
Yeah it does
help if you need to slow down like you said before junctions etc, although I'm pessimistic about any gains when going downhill - I reckon it's more economical to just freewheel (at a few hundred mpg usually), as if you leave it in gear, the car slows through engine braking, and you then obviously have to use some more juice to re-gain this lost speed!
Still find it ironic how we are on a forum for performance cars talking about hyper-miling techniques!! haha, mind you, every little helps when petrol is at £1.15 a litre!