The highways in Malaysia are good.
Specially the North South Highway from the Thai border till
Johor Bahru is very good. If you have rented car the best
way is to buy a good road map in Malaysia. You 'll have the
latest edition and it's cheaper than in your own country.
Any good shopping mall in Kuala Lumpur has a bookstore, which
sells a road map.
At the east coast is not a highway, but the road is also very
good to drive.
Right through the jungle goes a road from Kuala Lumpur to
Kota Bharu. A friend of mine, who used to drive this stretch,
wrote me: