If someone puts on larger auto tires than the vehicle was originally
equipped with, will the speedometer read faster or slower than the
actual speed?

Yes.

Usually, when you increase the size of the wheels (often called "plus one"
or "plus two") the new tires have a lower profile. In other words, the side
wall of the tire is shorter than the original tire, so the outer diameter
is almost the same.

Basically, you end up with more wheel and less tire. This can improve
performance as there is less side wall flex in the tire during cornering.
The flip side is it also makes the ride firmer since there's less tire to
cushion bumps.

Of course, you see people pushing this idea to ridiculous extremes now days
with huge wheels and extremely low profile tires. Personally I think it
looks stupid, but to each their own...

I cant seem to comprehend how to determine this.....

There are plenty of calculators online that will let you determine the
outer circumference of various wheel and tire combinations. Usually you
want to stay within 2-3% of the factory size to prevent issues with the
speedometer reading faster or slower.

Here's one you can try:

http://www.1010tires.com/Tools/Tire-Size-Calculator

Anthony Watson
www.watsondiy.com
www.mountainsoftware.com