can someone point me in the direction of a speedo calculator so I can see what my actual speed is with bigger rims?

 

It should be the same or very very close if you keep the same diameter of the entire wheel/tire. Really, what you need is a wheel size calculator.

For example: On my car, I went from 14" wheels to 16" wheels. I figured out what the original wheel/tire diameter was, then figure out what size tires I needed on 16" wheels to keep the same diameter. The result is that my car is only like 1% different from original diameter.

Try this:
http://www.miata.net/garage/tirecalc.html

Put in your current tire size. Input the wheel diameter that you want to go up to. Then, play with the tire size to get the diameters as close as possible. That will tell you what size tire to put on your new wheels, and will tell you how far off the speedometer will be.

Or, get a GPS. My GPS shows my actual ground speed. Even on stock wheels, speedometers tend to read more inaccurately the faster you go. When I'm going 30mph by speedometer, the GPS reports 29. When I'm going 70 (speedometer), the GPS reads 65. When I'm going 80 (speedometer), I'm really going 73. All of my Hondas do this, even on stock wheels.