Whats the techinal reason behind putting 17 inch alloys on a car (any car), apart from looking flash?

Bal

 

You can have wider tyres without having to increase the profile width or reduce the overall wheel/tyre diameter.

 

By plus sizing you can decrease the depth of the side wall, which in turn allows for less deflection of the side wall during cornering. Benefits of this have to offset against increasing the unsprung weight of the wheel/combination.

To demonstrate that it works look at the Impreza WRC in tarmac spec. It wears 18in rims. Its nothing to do with its appearance - just have a look at the same car in ice/snow spec with its ridiculous thin and tall looking tyres. Its what works in the conditions the car is driven.

 

One of the reasons for having larger wheels is to allow bigger brakes to be fitted.