Just changed my Bridgestone Potenza RE050 for 4 Goodyear F1 Eagle GSD3 XL.

Above 50mph the steering has developed a vibration which not serious was definately not there before.

Could this be because the new tyres have a stiffer sidewall, or just that they haven't balanced the wheels properly?

 

I'd go with balancing or the nuts aren't tight!!!!

Could also be a slightly buckeled wheel that was on the back and is now on the front.