Many people think that fitting big alloy wheels will improve the handling of their car. The vast majority of these people will be incorrect, it is far more complicated than that.
Adding big alloys usually adds weight to the car, more specifically unsprung weight which is the worst sort of weight you can add, as it increases the demands on the suspension and so worsens the handling. Light wheels are the best but are often expensive.
Fitting larger wheels often means fitting wider tyres. This changes the handling of the car. Notice I said changes the handling rather than improve. Most people perceive fitting big alloy wheels with the wider tyres as an improvement as the steering will feel more responsive and the turn-in is sharper due to the smaller tyre sidewall. However that isn’t the whole picture. As I’ve said earlier, larger wheels usually weight more which has a negative effect on the handling. The tyre also plays a large part in the suspensions compliance too and so fitting rubber band type tyres rather than big balloon type ones will decrease the comfort and increase the noise in the car. You also tend to get less feel with big wide low profile tyres, i.e the transition between grip and slip is more sudden and so they tend to be harder to control on the edge of adhesion. Also due to their wider shorter footprint on the road they are more prone to aquaplaning and slip on mud/snow. If you take this to the extreme take a look at rally car tyres on ice, they look almost like bicycle tyres they are that thin to increase the pressure on the ice in the contact patch.
I myself prefer the way my girlfriends car (220 turbo) handles on 195/50/15’s than when it is on 205/40/17’s, although ask most people on the street which looks and drives better and they’ll say the 17’s as they look “better” and the turn in is sharper with the 17’s. There is a huge weight difference between the two wheels and as a bonus the 15 inch wheels have significantly cheaper tyres!
Tyres and rims can make a big difference to how your car feels and handles.
Many people will say fit some larger rims and it’ll handle and look better. Well I can’t comment on how it’ll look but in terms of wheels bigger isn’t always better. Whilst bigger wheels means lower profile tyres which will make the car feel like it turns in faster, it also means usually that you have more unsprung weight at each corner which hampers handling. It also makes for a far less comfy ride. As an example I have 2 sets of wheels for my girlfriends 220 turbo petrol. One are standard 15’s and the other 17’s. The 17’s make it feel like it is on rails but the ride is harsh and it handles bumps and uneven surfaces less well. The 15’s are comfier, and more forgiving if I start to loose grip. They are also a lot cheaper to buy tyres for! As for tyres well its down to personal preference and budget. I’ve driven on budget “ditchfinder” specials, and also high performance tyres. Providing you know the limits of what you’ve got then that is the main thing.