Different size wheels for test purpose

[H17YD L]

Hi,
I'm having trouble with what seems like a wheel out of balance but disappears after a short drive on the motorway. It's been into Audi and they have said its a tyre out of shape.
I'm picking up some wheels tomorrow that I will be using for winter tyres this winter, they are 17's but I want to know if I would be ok to stick 2 17's on the rear with the original 18's on the front to see if the vibration disappears altogether the difference isn't that much see table bellow. But I'm not sure if it will cause the quattrro and problems?

Before anybody suggests to put all 4 17's on I cant as 2 tyres are scrap

 

[Ash187]

I can't see why it would be an issue as long as you use those tyre sizes... If its for test purposes it should be fine. Think of what happens when you fit the spare wheel onto the car, they also are a different size but you can still drive the car with them just as long as you are not planning on driving it like that for a long time

 

[H17YD L]

Thanks mate I thought about the spare scenario and that doesn't cause a problem! So I'll go with that
Cheers!