Most probably, not many people will travel from Udaipur to Jamnagar unless they have some work in Jamnagar, specifically refineries of Reliance or Essar. But if some one wants to to Jamnagar from Udaipur in shortest time and low cost with comfort, best is to go to Abu Road Railway station and take a overnight train to Jamnagar.
I traveled to Jamnagar on 17th July 2012 from Udaipur. We decided to drive down to Abu Road by car, park the car at Abu Road junction for 2 days and come back from Jamnagar, pick the car and drive back to Udaipur!
NH76 is in very good shape and one can easily drive at 120KM per hour. This four lane high way has less traffic. Look out for falling stones at some places though!
We started from Udaipur at 5:30PM and reached Abu Road by 7:15PM. We stopped at couple of places to take pictures.
The train was at 12:30 at night. So we stayed in a hotel at Abu Road town near railway station. Normally, trains in this part f India runs on time. So, do not reach station late else you might miss your train!
Jamnagar is small town but very famous for Reliance and Essar Refineries. The refineries are located around 35KM outside the Jamnagar town.
Jamanagar has small port, Lakhota Lake in middle of the town and Ranjit Sagar lake which is outside the town.
Here are few pictures:-