I own a Ford Focus 2.0 Titatanium on an 05 plate, it has 205/50 R17s fitted. I am travelling to Germany in the new year and I'm going to buy some winter tyres and some steel rims. I've found some but they are 205/55 R16s and there will be a slight difference in the rolling circumfrance of 17mm! Does anyone know if this will affect anything? The spare is on a 16" steel rim but is a 125/85 R16 so I know that the rim will fit over the calipres etc.

Try something like this:

http://www.tyresave.co.uk/tyresize.html

Ideally you want the rolling radius to be as close as possible to the original tyre, otherwise your speedo will be wrong. There will also be slight changes to ride height and overall gear ratios.

As a tyre wears out from new to old the radius will change anyway, so it's not that big a problem

But a general rule of thumb is to keep the dimensions within 1% so you should be fine (your new tyres will be about 0.75% difference)

Try something like this:

http://www.tyresave.co.uk/tyresize.html

Ideally you want the rolling radius to be as close as possible to the original tyre, otherwise your speedo will be wrong. There will also be slight changes to ride height and overall gear ratios.

As a tyre wears out from new to old the radius will change anyway, so it's not that big a problem

But a general rule of thumb is to keep the dimensions within 1% so you should be fine (your new tyres will be about 0.75% difference)

Pitmonster very useful (link) cheers!

The difference in circumference of 17mm that you mention in your opening post is less than the difference between a brand new tyre (tread depth 8mm) and a very worn tyre (tread depth 2mm)

e.g. my Focus has 205/55R16 tyres.

When new, with a tread depth of 8mm they have a diamater of 837mm and a circumference of 2629mm

When worn out, with a tread depth of 2mm, the total diamater has reduced by 12mm (6mm on each opposite side of the tyre)

The diamater has therefore reduced to 825m (total change 12mm) and the circumference reduced to 2591mm (total change 38mm)

This is a reduction of 1.4%

So the tyres you are suggesting are well within tolerance and you should have no problems with them.