I ve tested several times a very good malfunction in the box: Volcanic ash. It is a nasty one because both engines are not usable and all your pitots are blocked. 777 is a democracy, if 2 out of the 3 pitots are blocked the ADM that are in majority reporting the same speeds are "winners into the election" for best data. In other works you will be prompted with erroneous speed for a while, till you hit 80 kts IAS and then the last ADM will be "elected". Back to that malfunction, your best friend would be the weather briefing with winds (predicted) and GPS ground speed since you cannot apply pitch, power, performance trick.