I always use the northern exit which brings you down into the retail park area at Auchan.
Then take the western bypass to take you to the circuit.
See here
http://www.the-twats.com/LE MANS As you approach the end of the Autoroute (A28) just north of Le Mans the dual carriage way splits in two, stay in the left lane signposted Le Mans and Tours, (DO NOT take Le Mans University exit on the right). Just after this first split you will then see another split in the road, take the right hand exit which flows around to the toll. On exiting the toll 320 (2004), you will come to a roundabout with a McDonalds and a Buffalo Grill, bear left around this roundabout and follow the signposts to Le Mans (N138) for 2kms. On the N138 towards Le Mans you will go past another McDonalds on your right, at this point stay in the right-hand lane, and take the right fork sign-posted Toutes Directions (E502/N138R), DO NOT FOLLOW THE SIGNPOSTS TO LE MANS CENTRE.
E502/N138R Ring Road - You will then be on the ring road signposted E502/N138R Toutes Directions, around Le Mans. Once on the ring road start looking out for signposts to Angers and Tours. Ignore any signposts to the circuit at this stage, until you go past the Champion supermarket on your left-hand side in approximately 7kms, then keep in the right-hand lane from now on as shortly you need to turn right at a set of traffic lights (D315), signposted to the Aerodrome.
(If you miss this turning, then take the next right, which is signposted to Tours E502/N138, which eventually goes onto the Mulsanne straight. This road will bring you past another McDonalds and a Carrefour supermarket on your left-hand side. Continue on this road straight over the lights and when you have past the Green Seven Hotel on the left-hand side you will need to turn next right (Rue du Panorama) just before Tertre Rouge Corner, signposted to Tertre Rouge/Expo campsites. Go past the Tertre Rouge campsite on your left, and then at the T-junction at end of this road turn left. Go past the museum and grandstands on your left, the airport should be visible on the right.)
Follow this road for approximately 2kms towards the Aerodrome, at the next major roundabout you come to (where you should be able to see the Airport and grandstands in front of you in the distance) take the exit signposted to the circuit (I think its the third one, N23) not the exit to the Aerodrome or Arnage. This takes you over a fly-over, which bears right round and brings you onto a dual carriageway (N23), stay in the right-hand lane and take the next exit right towards the circuit (D139). At the crossroads opposite the Main Circuit Entrance turn right onto the D139, continue past the grandstands on your left and you will then see the Bleu car park/campsite entrance on the right.
thanks to
http://www.the-twats.com/