Traffic, terrain, conditions, the car I'm driving, and the timeframe go in the mental blender and a speed of travel comes out my right foot...
I learned back in the 55 days to constantly scout for LEO/revenuers, and to not drive stuff that isn't capable and in good order.
I've been towing @ 75 in TX in the past, and been whistled by a car transporter who was likely +15 on me.
There's lots of areas where +10 or so goes un-noticed.
Some parts of the Country, I've put it @ 100-130.
Other parts, I'm happy to just cruise through and enjoy the scenery.