My suggestion for someone having difficulty keeping their speed in a 20% window was to aim 6 mph slower at highway speeds. That doesn’t constitute “far slower”.

My suggestion for someone having difficulty keeping their speed in a 20% window was to aim 6 mph slower at highway speeds. That doesn’t constitute “far slower”. In the Uk, if you are incapable of handling your car on a highway with other vehicles driving 12mph slower than the limit, then you are incapable of driving on highways - some vehicles e.g. HGVs are restricted to these lower speeds.