If a highway has two lanes, the average speed in the left lane is higher than the average speed in the right, but the only major passing opportunities are on the right when there's an empty stretch (because no one merges right).
If a highway has >2 lanes, the left is never consistently the fastest moving lane, and almost every passing opportunity comes from moving right. It is not uncommon to find the right lane near empty, and it is then possible to blow by everyone in that lane.
The exception to the above is in areas with heavy merging, wherein the average American driver seems to not understand how to merge (either blocking one another, or coming to a stop as the merge lane begins). In this case, speed seems to increase moving left.