The height of the tyre, is proportional to the width.
That number 40, means 40% of the width.
IE, a 225/40 has a side wall that is 90mm
A 245/40 has a side wall that is 98mm
Once you start to factor everything in, that's a much smaller rolling distance, and it means for your car speedo saying 100KM/H, your car is going quite noticeably slower.
But I would say, you really need to convert the diff from auto to manual so that it'll all read correctly again, this would be the easiest and best way to fix your problem as well as changing tyres to the correct size.