MapQuest's map data is not all from NavTech. Some is from GDT (Geographic Data Technology), who also supplies the geocoding data used by MQ (as of '99, and as far I know).
Maps in large cities are generally NavTech (where it has better data), but those from more rural areas are generally from GDT. (This is in the copyright info on the lower left part of the map).
The speed limit is derived from the road 'class' (which is part of the street network data). So the road may have the wrong class. Also, they may be using 55 mph as the speed limit in areas where it is 65 (I seem to recall that being the case).
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MapQuest's map data is not all from NavTech. Some is from GDT (Geographic Data Technology), who also supplies the geocoding data used by MQ (as of '99, and as far I know). Maps in large cities are generally NavTech (where it has better data), but those from more rural areas are generally from GDT. (This is in the copyright info on the lower left part of the map).
The speed limit is derived from the road 'class' (which is part of the street network data). So the road may have the wrong class. Also, they may be using 55 mph as the speed limit in areas where it is 65 (I seem to recall that being the case).