I know, this is way too much info...
Just as one data point; here's a trip I did from Concord, CA to Salt Lake City and back just about one year ago; using I80 to get there and US50 (mainly) returning. I plotted out the Google times beforehand, and recorded the actuals on the trip.
First chart shows Google map times:
Concord-SLC: 10.25 hours
SLC-Fillmore: 2.25 hours
Fillmore-Eureka: 5.25 hours
Eureka-Concord: 7.5 hours
Here's what the timer in my car recorded as actual "ignition on" time:
Concord-SLC: 10.25 hours (straight thru leadfoot driving)
SLC-Fillmore: 3.4 hours (some time lost in SLC looking for a place to eat dinner)
Fillmore-Eureka: 6:00 hours (some time spent driving around inside Great Basin National Park)
Eureka-Concord: 9.25 hours (includes an hour spend idling on a business phone call plus looking for lunch in Fallon and Dinner in Truckee)
Bottom line: for my type of driving (never saw a speed limit I didn't think I could add 5-10mph to) the transit times in Google are pretty good. What messes you up are the stops/side trips/etc's that happen... you need to account for them.