If the speedo reads 30mph am I actually going nearer 32mph ?
If the speedo reads 70mph am I actually going nearer 74mph ?
No because STOCK tyres/speedo usually read between 5 and 10mph HIGH! For a bunch of reasons.
Just to begin with tyre sizes are nominal, not exact. Different manufacturers tyres vary by more than this! And 8mm of tread can wear off changing diameter by 16mm. Even inflation pressure varies this diameter. And by law a speedo must never read low. But ignoring all of this...
Almost all vehicle speedometers over read - often more so at higher speeds. And two identical vehicles may read differently too as these things are not very consistent.
The GPS in my phone is accurate. My vans speed used to over read quite a lot at low speeds and was almost correct at high speeds. The revese of what you usually find, but over reading all the same. After fitting "bigger" tyres it now under reads by about 1mph at 40mph And by about 2.5mph at 100+
That actually makes the error much
less than before... But in the opposite direction. Either way more accurate than you need.
What I am saying is that car speedometers are not accurate anyway. And the difference it makes isn't big enough to matter and may end up improving its actual accuracy...
Some start off more accurate than others. Test it with your phone, and then you decide if it matters. Anything up to a 5 percent error wouldn't make any practical difference to you would it? That would be just 1.something mph at 30mph town speeds. And you wont get a ticket unless you are significantly above the limit.
Read this. Fitting bigger tyres diameter usually just makes your speedo MORE accurate...
http://www.thecarexpert.co.uk/how-accur ... eedometer/