I'm looking at getting some 18" 'Snowflake' ST alloys. I have a few questions about this. What tyre size should i use on these? If my speedo at the minute reads about 3mph over my actual speed (according to those signs that say what speed you are going) should i be worried about any difference the new tyre size will cause? Where would be the best place to buy them from?

My current tyre size is 205 55 R16

Thanks

Most common size for an 18 would be something with a 40 profile (sidewall) so 225,40,18 is about the most common, then 235,40,18 wouldnt think you could go much wider than that. Speedo shouldn't really be affected with these tyre sizes.

225 40 18 sounds good and according to the calculators doesnt affect my speedo too much

Just type "tyre size calculator" into Google.

There's plenty of websites that do all the maths for you, you just type in your tyre sizes and it tells you how the two compare.

You want a variance as low as possible - less than 2% is what you should be aiming for. Any more than that and your speedo will be very wrong!