Changing of wheel size and speedometers?

Can somebody set me straight here? I was going to put some 17" Voxx FA's on my car, and I understand that I'd have to go down a tire size to keep the same wheel size. Ok, now this gets me thinking, I am planning on getting some lowering springs too and I want the wheels to fill up the wheel-wells so it looks nice. So I'm thinking, why not 18's? Then I realize that they'll be bigger than the 17's and that will change the speedometer (no?). Why can't you just keep the same size tire on the 17's and not go down a size?

Well I guess my question is, these people going around in Honda's with stock 14's that put 18, 19, 20, 22" wheels on their ricers, do their speedometers get messed up and if so, will it do anything to my Volvo if I go from stock 16's to 18's? If so, what can I do?

Heres what they are doing.

For every size bigger you go in wheel size (plus sizing) you have to drop a full aspect ratio in tire size (if memory serves).

So if you go from say a 195/70/14 (14" wheel) to a 195/40/17 (17" wheel) you should have the same overall height tire, but much shorter sidewalls and a bigger wheel.

Heres an online deal to figure how far off your speedo will be should you actually change your tire height. http://www.4lo.com/4LoCalc.htm

As for lowering the car, I'd do your wheels and tires first then you'll know exactly how low you can go.

Thanks.

I was planning on putting the wheels on before lowering it (though I am going to get them at the same time, obviously the wheels will stay on for at least a few days before I lower it. I was thinkin 1 1/2 or 2, that is all I can do, prob 1 1/2).

Anywho, answer my other question (about wheels fitting..)

I'll go with 17's too. Sh** the worst it would be ...I'm going 75 on the highway and it's actually 79. Not bad at all.

55 is 58. Meh. nothing. and if I go down a tire size then it's not a problem anyway.

But then the wheel doesn't fill up the wheel well as much, meh.

This is true. Look at the project 242ti on this site. I went from 60 to a 50 series tire and then dropped the car using IPD's springs and it fills up the wheelwell really well still.