My "safe" is the speed limit when logical and the prevailing speed the traffic is moving when congested. We use interstate 80 through the mid west a great deal. In the early morning I can drive 60 mph. However by 10 AM the traffic is running 70 to 80 mph. I follow the normal flow which is likely 70 while the 80 mph cars pass me. If I can find a few governed semi tractor-trailers I will follow them at 62 mph. I do not want to be the road block that gets to fix the rear end. My sweet spot speed is about 62 mph at 1600 rpm towing a Jeep Wrangler. I do not bother with mph calculations because they do not matter - you either go or you stay home - choose!
Glen Burgess
2004 Country Coach - 42' tag
2013 Jeep Wrangler Unlimited