It is 167 KM appx........The road from Trichy to Velankanni is full of bypass sections and you never enter any major town. That means finding a good hotel for food/rest is difficult. It is better to get into one of the towns like Tanjavur for rest/food. Another interesting aspect is the sheer number of railway crossings! Following are the major towns and the approximate distance from Trichy,

Plz refer the following website for details.:-

http://www.jayson.in/travel-reviews/velankanni-shri ne-basilica-travel-report-trivandrum-to-velankanni-via-trichy.htm l

Trichy, Vallam(45km), Tanjavur(20km), Ammapettai(20km), Needamangalam(19km), Tiruvarur(20km), Nagapattinam(30km), Velankanni(12km). I recommend you take a break at Tanjavur or Tiruvarur.

Browse mapmyIndia.com. you will get all the information not only for the desired route but also its nearby places of your interest

The road from Trichy to Velankanni is full of bypass sections and you never enter any major town. That means finding a good hotel for food/rest is difficult. It is better to get into one of the towns like Tanjavur for rest/food.resting ! Following are the major towns and the approximate distance from Trichy,
Trichy, Vallam(45km), Tanjavur(20km), Ammapettai(20km), Needamangalam(19km), Tiruvarur(20km), Nagapattinam(30km), Velankanni(12km). I recommend you take a break at Tanjavur or Tiruvarur.
Just walk to nagapatnam on NH 67