Years ago, pre autoroute we used the Calais, Abbeville, Rouen route and thoroughly enjoyed the relaxing drive through some lovely small towns and villages, stopping often for coffees and grub. If you want to be at Le Mans by lunchtime my advice is to set off very early. Whilst the toll-free route is a great experience it's quite a slow 262 miles. You do get lots of French families sat along the roadside offering encouragement to drivers of rorty machines to "give it some", especially on the Sunday after the race.
try viamichelin.com for a detailed route. Tick the "avoid tolls" box.
Enjoy the drive