Hello. I have recently put ST alloys (205/40r17) on my ztec s (195/40r16) and was wondering how much it can effect the speedo or will it need to be calibrated? Also wondering what size spare wheel I'd need because my spare wheel is 15 inch? Or will it be okay? 

It won't affect the Speedo enough to worry about.  Your originals should have had a 45 profile so the rolling radius is close.

The original spare wheel will be fine.

 

Okay thank you! 

The diameter will be 6.3% larger. Your speedo will read 70mph when you are actually doing 74.4mph. You will notice the slower acceleration especially off the mark. You need 205/35 or 205/30 to get nearer the original diameter.

36 minutes ago, btmaldon said:

The diameter will be 6.3% larger. Your speedo will read 70mph when you are actually doing 74.4mph. You will notice the slower acceleration especially off the mark. You need 205/35 or 205/30 to get nearer the original diameter.

The OEM tyres should have a profile of 45 rather than 40. In which case the speedo will read slightly below your actual speed by 2mph at 70. Not really worth worrying about since the speedo reads low anyway.

If you set your wheel size in forscan it will correct your Speedo for you.

ST alloys are quite heavy so you may notice an increase in fuel consumption.