First you've got to understand the numbers. First part is width, second part is the height as a precentage of the width.
So 195/70 is 136.5 tall and 205/65 is 133.25. So you have a total rolling circumference difference of not much- 2034mm vs 2055mm, around 1%, giving you a speedo error also of around 1%. (and a gearing change too) But, what this also means is that when it's about halfway worn out a 195/70 is roughly the same size as a 205/65 is when its new
But you also have to know 2 other things... One is that your speedo is almost certainly already inaccurate by design, it'll overread (by more than 1%)
The other is that not all tyres are the size they claim to be. I don't have numbers for car tyres but I did some measuring for motorbikes and the error rate was pretty considerable, as much as 5% off, so a bigger variance than you'll get with your change of size. Car tyres may be better.
Course, they're wider so you might want to check your rim width too. 10mm is not an awful lot but you never know.