MGF wrote:The car has 185/55/R15s at the moment but can also use 155/80/R13s.
Is the 13" wheel the standard size? Do you have wheels that size available? Tyres in the smaller size are certainly a lot cheaper, although the range available seems quite limited in both. If you were buying from mytyres, for example, I would suggest considering Hankook Optimo 4S, Vredestein Quatrac 2, Vredestein Quatrac 3 or Goodyear Vector 4 Seasons from the ranges they currently have listed.
If you are looking for the wheel and tyre sizes supported by your car there is sometimes a sticker with pressures for different sizes inside a fuel filler cap, or maybe the same information will be in the handbook. The reason for mentioning this is that there might be a 14" tyre that is suitable as well.
MGF wrote:I understand there is an advantage to skinnier tyres in the snow but how much of an advantage is there with all-season tyres compared with a proper winter tyre?
I understand the advantage of a narrower tread is primarily for accelerating and braking, which would be especially useful on compacted snow and ice, with the trade-off being less lateral grip which might be noticeable as greater susceptibility to cross-winds on motorways. Even the wider size is relatively narrow by todays standards.
A good winter tyre should be better than a good all season tyre in cold, wet, wintery conditions when both are the same size. However a good all season tyres should do almost as well in most of the wintery conditions we get in the UK. Similarly, it should do almost as well as a normal tyre in the warmer parts of the year. That's certainly the experience we've had on our cars.
MGF wrote:And what would be the difference when there isn't any snow?
Our car with Vredestein Quatrac 3 tyres sounded a bit 'squidgy' when turning the wheel in the summer however we didn't notice any grip defficiency.
there is only the road, nothing but the road ...