gate crashing a thread here, but as far as wheels and tyres go, you want them to be as light as possible. People will talk of unsprung weight and how it is important to keep it down. This means it takes less effort to turn them, speed them up or slow them down. It also means that you will get more feel through the steering wheel. The other important factor is that with a wheel you want the mass to be as close to the centre of rotation as possible - that makes it easier to turn.
So that's the science bit sort of.
Put into practice: light, smaller diameter wheels, light tyres. OK so small wheels look a bit daft, but a set of 16s with light tyres will make a significant positive difference to the performance of a car with big wheels and heavy tyres. Problem is with big wheels all the weight of the rim and tyre is further away from the centre of rotation and makes it harder to turn. So just to the tyres: oddly the big brand name tyres are significantly lighter cheaper tyres because they use better materials: For example a Good Year F1A2 in 225/45/17 weighs 8.7kg, whereas a Bridgestone S02 in the same size weighs 12.4kg - and that's not the heaviest. Some are nearly double the Goodyears weight. Other light tyres: Pirellis, Dunlops, Michelins (bit heavier, but not too much). Bridgestones for some reason seem to be made out of concrete, continentals I can't find weights for. However, some of the less well known brands are surprisingly heavy. Does it make a difference? Well from my own experience, going from a heavy wheel/tyre combo on my Giulietta 1.4tb (non-MA) to one that was 12kg per wheel lighter (I couldn't believe the difference either), mpg went from 32 to 37. if you have a less powerful car you are more likely to notice a difference. It's not conclusive, but then again weight matters - every manufacturer out there is looking to save weight on their vehicles so I suppose there must be something in it. I've just bought a new set of winter tyres for the wifes DS3 & I was talking to the owner of Kingswinford Tyres & he said everyone wants Goodyear F1A3s at the moment.
146 1.6TS, 156 2.0TS, 156 1.8TS, 147 1.6TS, 147 2.0TS, 159 1.9JTS, 147 1.6TS, 159 1.9JTDm, Mito 1.4, GT, Giulietta 2.0JTDm, Giulietta 1.4tb, Giulietta 1.6JTDm........