Speedometer accuracy... can it really be this bad?

I've always used the milemarkers on the road to get the odo as close as possible.
That might be the best thing for you to do here.
The odometer is directly driven by a worm gear off of the speedmeter cable.
If that's right, and the speedometer is off then something is wrong with the speedo.

On the tire, sometimes the manufacturer will also provide rolling radius. That will be a little more accurate than going from the diameter.