I used the iPad GPS to check my speedo. The faster the cars moving, the greater the error. For example in 5th @ 3K rpm speedo reads 5 under; at 3.4K in 5th speedo reads 15 under, etc

Low speeds under 50 are pretty accurate.

Can this be corrected at a speedometer shop ?

Yes, a good one can at least improve things. They'll probably change out the spring that returns the needle to 0; a stronger spring makes the gauge read less for a given speed, a weaker spring makes it read more.

--DD

Are you running stock tires? This can also effect your speedometer. North Hollywood Speedometer can adjust it.