Actually, a lot of erroneous data and info about GPS has been posted, and which company owns what, etc.
The real issue relates to the cost of data acquisition and Return On Investment (ROI) for such data. In addition, there are many potential liability issues if the information is incorrect. This is why good GPS database updates cost so much money. Lots of good GPS navigation databases contain speed limits, but they are rarely free.
Here's some brief history on GPS and the industry.
Having spent many years in this area, GPS used to have a significant amount of intentional errata, which has been eliminated since around 1999. That being said, the GPS databases out there are compiled by essentially one of two companies, each using their own unique methods of information acquisition.
That being said, there's really only one source that's truly reliable, and that's from Navigation Technologies Corp., or NavTeq. They've been around since 1985, originally from Germany, and they actually used low-flying aircraft to document roadways and locations, then drove each of the roads and documented such things as overpass heights, locations of mailboxes, actual addresses (not approximations) and other geographic data.
Imagine you're a long haul trucker traveling the path your GPS tells you, only to find out you can't make it under the overpass, and you have to take an alternate route that's hundreds of miles out of your way. Yes, that used to happen often, if the database wasn't from NavTeq. Likewise if you were to punch in a non-existent street address, the NavTeq database would know that address doesn't exist and the corresponding app might ask you if you wanted one of the nearby addresses.
I imagine most people can see why this road data can be pretty valuable information, knowing if someone gave you a wrong address or directions to a destination. But, speed limits are a completely different matter, often changing in states, counties, cities, and local regions way more frequently than the GPS database companies such as NavTeq can keep up with. Even Wiki's attempt to make speed limit maps open source failed rather miserably, years ago.
There are datasets out there that offer speed limits, such as the navigation within my Lincoln vehicle. However, it's only as good as the latest database update, subject to the accuracy of the data within it and, I'm not concerned enough to spend the hundreds of dollars to update the database. It works well enough.
The bottom line is this, if having speed limits for each and every road for a given area is important for you, expect to pay for accurate information, sometimes frequently. Even then, the data for speed limits may still be incorrect. As examples, the State of Washington just increased its max. highway speed limit in August 2015, Nevada and Montana are increasing their max. speed limit on some highways to up to 80 MPH, effective October 1, 2015. In addition, many states allow county, city, and even smaller communities govern and change their own speed limits, updating them as they see fit.
I don't plan on updating my database now in August, then again in October, just to maintain the correct speed limits for those states, especially when it's doubtful the regional speed limits would be correct.
So, until and unless someone invents an automated manner to collect and catalog the speed limits from each and every neighborhood community, city, county and state, between each and every waypoint, on a very regular basis, the speed limit information at anyone's fingertips will most likely continue to be constantly flawed. Hey, even GasBuddy has to rely upon people reporting gas pump prices as they sometimes fluctuate daily (or even hourly these days).
Having speed limits for every area may be a good idea, just next to impossible to maintain accurately. The speed limit information you rely upon may be very wrong, and could result in a speeding ticket and hefty fine. It's doubtful any judge or jurisdiction would waive any fines on the basis of relying upon any GPS navigation technology or database.
Even Google Maps only updates their Street View Maps every few years for many highly populated areas, and there could have been many speed limit changes in between. In lesser populated or more rural areas, they might not drive through for Street View updates for a decade or more.
Hopefully this can resolve the questions and concerns of people in this area.
Like 70MPH for motorways, unless driving a truck, bus or towing something, then it's 60MPH. 30MPH on single carriageway roads in a built-up area etc, occasionally it's 20MHH, which will be clearly stated on signs. Very likely similar for the US as well, and most other countries, including China.
