Changed my rear tyres to some goodyear f1 225/45 r17 recently leaving 205/50 r17s on the front. Doing some motorway driving today have noticed some quite marked vibrations above 70 mph. Still need to check if they are fitted for the correct direction. Are the wheels perhaps not balanced correctly? Or is because I am mixing sizes?

Cheers

They'll have some arrows on the side walls pointing in correct direction of rotation.

Sounds like the wheels need to be properly balanced though.

I did exactly the same (with Goodyear F1's) and ran for about 6000 miles with no problems. Go back to your tyre fitters and get the wheel balance checked.

Deffo sounds like wheels have not been balanced - if you paid for it I'd be pretty annoyed as it can be downright dangerous if you were to exceed NSL by a margin

Other things to check are tyre pressures as if one is too low it can have a pretty bad impact on the stability of the car when it hits bumps in the road etc.