92 Lumina speedo issues

[BigMikeyB]

My speedome... spedometr.... SPEED GAUGE THINGY always tells me I'm going faster than I really am.
While this is cool for impressing people and making them think my car goes faster than it really does, it's becoming a problem.

WHAT I KNOW:
The curve is exponential, only off by about 5 miles around the 30-40 region, but is off as much as 15 by the time I hit 55 (it says I'm going 70).
My wheels are stock and my tires are of appropriate size.
I can watch the needle fall even after I come to a slow stop (which tells me that it's not a mechanical thing but I could be wrong).

WHAT I WANT TO KNOW:
Is this a computer problem or a mechanical problem?
How easy is it to fix?
Is it worth fixing?
Why did daddy leave us?

If you can't answer that last one, I won't hold it against you.
Thanks in advance.