my coupe has 205 tyres on it
they look a little narrow
im just wondering when it comes to replacing them what is the best width tyre to get?
obviously they will give better grip if wider, but what about drag and fuel economy?

no they won't.....

Wider tyres change the contact patch *shape* but not the area

The ONLY ways to increase grip is to use softer compounds or to increase the force acting on the contact patch by increasing the weight (or even by increasing downforce - not easy on a Coupe!)

Optimal size on a 17" rim is 215/40

Also ive found the wider the tyre the worse grip in the wet?

It does increase the cornering grip because more of the contact surface is available for lateral load.
So you are sacrificing straightline grip though.

so 215 is the best size then?

JohnS! You have a pm!