Google Maps plus some satellite images is usually pretty good, and of course nothing beats being prepared. (Bring MUCH more water and sun protection than you think you need!)
To give some examples, in the past twelve months or so I've navigated by GPS (using Google Maps to visualize my position) in mountains in Iran, semi-rural Ukraine, forests in France, middle-of-nowhere Canada, and forests in America. In case you think that my GPS worked just because I had good devices, I've used everything from a nice iPhone 6+ to a decent Galaxy S5 to a crappy Huawei P8 Lite which I bought for $65 used.
I'm not sure how you can quantify GPS as "80% accurate". Does that mean 20% of the time it puts you in the next state over? Or that it's 200 meters off? I can't think of a reason why your GPS position would be wrong... it's not as if physics just breaks down in the Rockies.
In order to get such tremendously poor results, I have a feeling that you did something like turning off GPS (which forces your phone to rely on cell tower triangulation) or using GPS where it doesn't work (like in a cave.) If the actual, raw values from your GPS were wrong I would be extremely surprised.
Google Maps can be pretty spartan in wild parts of the US.
In some cases, I can tell they used old data. I know one road in particular that did exist 30 years ago, but the forest service deliberately closed off when a rock quarry closed, and it has been re-forested. It still shows on the maps. Another that shows on the maps was planned to be built by the BLM, but never was... yet the map shows it. So I'm not sure exactly what data set they built their maps from, but clearly it did come from data, not actual exploration... because the dirt roads in the American west are just not accurate.
But it's also the case that dirt roads range from well-maintained and graded to you'd better have a jeep and know how to drive it--and the distinction isn't always obvious on a map (digital or otherwise). Road conditions also vary based on season, weather, and how recently there's been maintenance work done.
Someone stated in an earlier discussion that part of the issue is that a lot of this dirt road data is from US Census TIGER data, which was never intended for automotive navigation. [1]