How is taxi speed measured?

What is the maximum taxi speed and who defines it? has a lot of interesting thoughts and facts about what maximum taxi speed is admissible or practical. It brings me to this new question: how does a pilot know how fast he/she is going when on the ground, apart from just looking out the window?

Afaik most planes only have an airspeed indicator, which I suppose is not of much use while taxiing due to surface winds being in the same order of magnitude as ground speed. Also I wonder if an ASI has any usable accuracy near the bottom of its range. The same might be true of GPS speed indication.

Did some more research and found out GPS is much more accurate than I thought it was. http://www.ncbi.nlm.nih.gov/pubmed/15519597 and other sources find it better than 0.5 ms-1 most of the time. I guess that is better than what can be calculated from wheel speed, especially with big, fast wearing tyres.

• On old 747-100/200 the ground speed was shown on the inertial navigation systems, and it was accurate at taxi speeds. These days I'm sure the glass cockpits can display the ground speed. It would certainly be accurate just as your car's GPS readout is accurate at low speeds. Looking out the window can be a surprisingly INACCURATE way of judging groundspeed, especially if a pilot has just transitioned from an aircraft where they're close to the ground to one where their eye level is over 30 feet up. – Terry Aug 16 '16 at 20:52

• These days in most aircraft, GPS units are standard. <-- Citation Needed! (seriously, I can walk down the ramp and well over half the aircraft don't have a panel-mount GPS). They're becoming more common, certainly, but they're far from ubiquitous. Of course in 5 years time I'm pretty sure that statement will be correct :) – voretaq7 Aug 17 '16 at 17:33