wouldn't it depend on whether the overall diameter changes or not? if he's going from, say 16" with 55 height ratio to 18" with 40, then you would need to do some maths to determine whether the overall dimensions differ or not.
or am i talking balls? :tonofbricks:
If you want to fit bigger alloys without affecting the mileometer, speedometer, gearing, etc you have to make sure that the new alloys and tyres retain the same overall diameter as the old tyres. If you want to know how close to the original diameter the new alloys and tyres will be then log on to www.tyresave.co.uk and use their tyre size calculator to compare the difference between the old tyre size and your proposed new tyres. That way you can get the new tyres as close as possible to the overall diameter of the original tyres, and it won't mess up the readings on the dashboard. If you just choose a lower profile tyre without doing your homework and fit them to the car then there is a good chance that you could mess something up. Always check first - that way you'll stand more chance of getting it right and avoiding any problems.