Kuala Lumpur (Malaysia) to Sumatra (Indonesia) by roro ferry

I found this website may be this could help transport a car from Malaysia to Indonesia.

http://www.gbgindonesia.com/en/services/directory/2014/asdp/introduction.php

The name of this ferry company company is ASDP Indonesia Ferry

They have a transfer from Penang (Malaysia) to Belawan (Sumatra-Indonesia)

Also in their route map they have good international transfers network within Thailand-Malaysia-Indonesia

Yes and no.

As someone who did a similar research, and have been to Dumai, Belawan, and Medan in Indonesia (Medan is the city, other two are ports), plus the Port Klang in Malaysia, I'm still not sure how they work.

There used to be a ferry service between Belawan (nearest port in Medan city) and Port Klang (both ports are about 1 hour from main cities Medan and Kuala Lampur), but it does not run anymore. The best link you are left with is the Port Klang - Dumai (Riau province in Indonesia) route.

I don't know which city you are planning to go in Sumatra, but if you are heading Medan, I highly discourage you from doing it. The roads are quite poor, and Dumai - Medan is a painful 10-12 hour drive. It is cheaper to simply fly there (and rent a car in Indonesia if you need one), and you'll save a day as well. The ferry schedules are hard to find, and some ports have additional charges that you have to pay separately, etc. The ferry will only take 3 hours, so there is one bright side to it.

It is better to call the ferry company first, because most of the online information are quite outdated, and some ferry lines are no longer working because the popularity of budget airlines such as AirAsia. A KUL-KNO flight usually costs under 100 RM, and a ferry ticket alone costs about the same. When you factor in the transport costs to/from ports, taking a flight is the cost effective and faster way.

Update: Quoting Wikitravel Port Klang:

To/from Dumai: There are several operators and agents selling tickets at the terminal

• Indomal Express/Malaysia Express by Doyan Shipping (Tel: +60-3-3167 1058). Departure times written on signboard at ticket counter, usually about 9am. RM100 one-way. Journey takes just under 3 hours.
• MV Pelita Jaya Express/Sabang Marindo II by NKH Ferry Services (Tel: +60-3-3166 0122). Daily departures at 10.30am. RM80/150 one-way/return.