Kyle99 Posted May 6, 2019 Share Posted May 6, 2019 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? Quote Link to comment Share on other sites More sharing options...
TomsFocus Posted May 6, 2019 Share Posted May 6, 2019 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. Quote Link to comment Share on other sites More sharing options...
Kyle99 Posted May 6, 2019 Author Share Posted May 6, 2019 Okay thank you! Quote Link to comment Share on other sites More sharing options...
btmaldon Posted May 6, 2019 Share Posted May 6, 2019 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. Quote Link to comment Share on other sites More sharing options...
Luke4efc Posted May 6, 2019 Share Posted May 6, 2019 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. Quote Link to comment Share on other sites More sharing options...
Lebadoo Posted May 7, 2019 Share Posted May 7, 2019 If you set your wheel size in forscan it will correct your Speedo for you. Quote Link to comment Share on other sites More sharing options...
ThaiFiesta Posted May 7, 2019 Share Posted May 7, 2019 ST alloys are quite heavy so you may notice an increase in fuel consumption. Quote Link to comment Share on other sites More sharing options...
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