MaxMind tests the accuracy of the GeoIP2 and GeoIP Legacy Databases on a periodic basis. In our recent tests, the downloadable databases were 99.8% accurate on a country level, 80% accurate on a state level in the US, and 68% accurate for cities in the US within a 50 kilometer radius. For more details, see GeoIP2 City Coverage and Accuracy.
The GeoIP2 Precision Web Services contain our most accurate data at a city/postal code level, geolocating 7% more IPs the correct postal code and 3% more IPs to the correct city in the United States than the GeoIP2 Downloadable Databases.
Please note that we unfortunately cannot guarantee 100% geolocation accuracy. Accuracy exhibits high variability according to country, distance, type of IP (cellular vs. broadband, IPv4 vs. IPv6), and practices of ISPs. In favor of transparency, we publish various filters and granularity levels on our GeoIP2 City Accuracy page.