Malaysia, rich and developed as it is today, was mostly jungle until the 20th century, aside from a couple colonial towns on the coasts. That means: aside from those colonial towns (Georgetown and Penang), Malaysia is made for the automobile. The capital, Kuala Lumpur, a tiny mining settlement until the 20th century, is one of the few cities in Asia where driving is not a terrifying experience (although they drive on the wrong side of the road). And, thanks to the oodles of expressways, even the traffic in Kuala Lumpur is generally not too bad by world capital city standards.
There’s a toll system on the expressways: you pick up a card called Touch n Go, sold at most convenience stores, which is a unified card used for tolls nationwide, public transport in Kuala Lumpur, etc. — kind of genius. (Imagine a US city being able to do something like that?) The tolls are not expensive but Malaysians are price sensitive so you will find a lot of vehicles, too many vehicles really, using the two-lane roads instead, which means even thought Malaysia is fairly rich there are some unpleasant third-world vehicle situations to contend with (there are tons of logging trucks everywhere, goodbye rainforest!). Where possible, take the expressways and avoid those two-lane roads, even if it looks like (and Google Maps says) it will be marginally shorter.
One great thing about Malaysia, from the point of view of ignorant foreigners such as myself, is that most educated Malaysians speak English. The population is broken down between South Asians, Chinese and Malay (a narrow majority), and although Malay is the official language, in order to be polite many educated people use English between each other.
The one downside is that because it’s such a good place to drive, car rental is kind of expensive, especially around holidays. Nobody drives stick so the usual trick of renting a manual to get local-ish rates doesn’t work. You can get cheaper rates by going for a local car provider; I used Hawk, which has a very good website and good service. Note that returning the car at Kuala Lumpur airport is tricky, as of 2018 you put it in a packed parking garage and must find the spaces of your rental company which aren’t clearly marked, so allow a bit of extra time.
Cracked windscreen from leaving the car outside overnight in the rainforest, maybe a monkey did it? See insurance