well the weight of a nickel is about 10 grams. a truck cam do 12 tonnes for a six wheeler or 28 tonnes for a 22 wheeler with 3 axles. thats 600k coins for the 6 wheeler or 2.8 million for the 22 wheeler. am in Kenya so my calculations may not make complete sense, but I estimated for a fact you can't pack metal past 75cm high despite size of of vehicle. thus totally wrong.
Thanks for the comments. It is actually a space and volume problem, as you cannot be sure that the weight of the coins can actually fit in a truck either, unless you've made both calculations.