Garmin Gps Milwaukee

Which mapping is most accurate for drive times: Yahoo Maps, Google Maps, Mapquest, or a Garmin GPS?

Traveling from west of Milwaukee to west of Denver and am encountering drive times varying from 13 hours 20 minutes to 19 hours 38 minutes … some difference in routes. Which is actually the fastest and why such a great variance?

I’d go with Google maps , about 16 hours. It’s interstate all the way. I-80 is the most direct.