On a new car with the correct new tyres at full tread depth it is illegal for a speedometer to under read. They can over-read by a varying amount, roughly equivalent to 5% - 10% and most over-read by around 5%. In your case, this may be due to the wheel and tyre size. There are three 'official' sizes: 195/65 R15; 205/55 R16 and 225/45 R17 . You can use tiresize.com/comparison/ to check the rotational differences. I just did and 195/65 R15 and 225/45 R17 are exactly the same. This is for new tyres, of course. As they wear, rotations per mile become greater and so does the speedometer over-read.