Why does altitude from the SPEEDBOX differ from my consumer GPS?
The SPEEDBOX measures altitude above the WGS84 ellipsoid. This is a globally specified ellipsoid, that is a best fit for the entire Earth, and the standard reference for GPS systems internally.
However, the Earth is not actually a true ellipsiod, but a geoid which has undulations for many reasons, such as the weight of mountain ranges pressing down on the crust. As a result, mean sea level is not consistent thoughout the world, and local mapping agencies, such as the Ordnance Survey in the UK, will reference height to a geoid based on a locally specified mean sea level. This will differ from the WGS84 ellipsoid, and the difference is known as the geoid separation.
Most consumer GPS units output height relative to local mean sea level. The SPEEDBOX outputs height relative to the WGS84 ellipsoid (essentially a best fit to global mean sea level). Hence the two may differ, but neither is incorrect.
Further reading can be found here: http://www.esri.com/news/arcuser/0703/geoid1of3.html