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If you are determined to reach Munnar in record time and you are traveling in a reasonably good car, it takes about 4 hrs from Ernakulam which is approximately 136 km away. The Ernakulam - Munnar route extends through Thripunithra, Muvattupuzha, Kothamangalam, Neriamangalam, Adimali and Pallivasal.
Distance Chart:
Munnar - Ernakulam (135km) 4 1/2 hours
Munnar - Thekkady (110km) 4 hours
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will take another 30 to 45 minutes to get out of the city in the traffic.
These lines are from Wikitravel.com
Munnar is 140 km (about 3 hours) from Kochi. Remember to budget a good 6 hours if you are driving up or down, since the road is blind in some places and there are a lot of trucks and buses that seem to drive as if they own the road. Although the road is smooth (NH 49), it is narrow, twisty and curvy. Make sure you have a litte meal before you drive to Munnar on these roads, otherwise the next thing you will see on your dash-board will be your last meal! Kothamangalam is on the way and is 80 km away from Munnar. Between Kothamangalam and next town "Adimali", you can experience a beautiful cool drive through a natural forest. Alongside, you can see many refreshing waterfalls.