i have an empty space on my dash where a power mirror switch usually goes. to fill it in, i want to make a neat little shift light thing. i dont really need a shift light, i'd just like to rig one up as somethin cool to have. here's my plan:

3 little LEDs -- one red, one yellow, and one green.
all three set to light at different RPMs -- red: 100 rpms -- yellow: 5000 rpms -- green: 6800 rpms (redline)
one/off toggle switch -- so that i can have them all working, or none.

if you guys understand right, then you'll get that im trying to make it look like a stop light, where the light either changes, or the next light comes on at higher RPMs. heres a pic so u can get a better view of what i plan on doing.


now here are my question:
How will i hook these LEDs up to know what RPMs my car is revving to?