Smaller diameter tires do affect your speedo, also, may affect your abs sensors if you have abs brakes. Im guessing you are trying to use up a set of existing tires, otherwise I see no sense in changing tire size.
If you want to purchase other tires, here's how you do it:
example: 235/70R15
235 is width of tire in cm + profile in cm, 70% of that total is profile. 15 is rim diameter.
Therefore, if you increase 70 to 80, tire width gets narrower as profile gets larger. If you increase 235 to 245, and keep 70,same thing. You can maintain your tire diameter if you increase 235 to 245, but reduce 70 to 65. Get your calculator out, you will see the difference is negligeable. I've been in the auto parts business for years.