I see a lot of questions about tire and wheel size; I have a lot myself. My buddy orangecrush showed me the attached website to answer all my questions, including speedometer changes. You can enter the information for your stock tire, and then see how it compares to other sizes. This may already be posted somewhere, but if not, here you go:

http://www.wheelsmaster.com/rt_specs.jsp

 

That thing is pretty sweet! Way better than some of the other ones I've seen in the past.

 

Nice site!

Here are a few others......
http://www.discounttire.com/dtcs/infoTireMath.dos
http://www.1010tires.com/TireSizeCalculator.asp
http://www.miata.net/garage/tirecalc.html
http://www.dakota-truck.net/tirecalc/tirecalc.html

 

I prefer the wheelmaster tire and wheel calculator.

It does both tire and wheel at the same time and you can see the changes as you make adjustments.

It does tire sizes at the bottom.

We need a visual or 3D suspension calculator like that.

 

While shopping for new wheels/tires for my MINI, I used most of the calculators shown in order to get an idea of how much my speedometer would be off. My conclusion after running the 205/45/17 stock tire size vs a 215/40/18 through the various calculators was; your guess is as good as mine. I don't know that I would put much faith in the figures shown on any of them.

Experiment with these figures for yourself:

Stock Tire 205/45/17 vs New Size 215/40/18

Run the first set of numbers on the 1010 Tires calc, and do the nest test on the Miata calc. Notice the difference in speed at 60mph.

 

About 8 yrs ago I created an Excel spreadsheet that does the pretty much the same thing. At the time there was just one site that has a wheel/tire calculator (I think it was a Miata site), but it only accepted one tire size at a time. So I created a spreadsheet where you can enter the OEM wheel and tire size, and you could also enter up to eight different wheel-tire combo you're considering. It will then give you all the values of total diameter, wall height, rolling circumferance, and the difference compared to OEM, plus speedo difference.