Is there any way to quickly tell if your speedometer is accurate? ... or there anyway to calibrate your speedo? It seems as if a lot of cars are passing me on the highway.

Get on the interstate and set the cruise to 60. Then when you get to a mile marker start the timer. Drive for 3 mile markers. If you reach the 3rd marker before or after 3 mins you will know if your off.

Borrow a GPS if you don't have one. They usually have speed on them.

As for adjusting, there really isn't a good way. Add on that it'* probably accurate to 50mph and off from 60-80mph.

Find a cop running radar. Ask if he/she would mind checking your speed against your speedometer. I've done it and they were surprisingly pleased to do so.

Here, we have a speed radar on the highway just before the bridge. No cop or anything, it just informs the users of the highway of their speed, and the limit. Its pretty cool if you can find one in your location.