Adding fixed costs automatically increases the overall costs. If you want to compare it with the Solomon instances, you need to subtract the fixed costs, i.e. minus number_of_vehicles * fixed_costs. But still, you might not end up with the best number of vehicles. The reason is that the algorithms in jsprit do not minimize the number of vehicles first and then the total variable transportation costs, but minimize variable transportation costs right from the beginning. Sometimes, however, these are competing objectives. Here, you can always try to build an approach yourself. For example, start with an infinite fleet. Solve the problem. Turn it into a problem of finite fleet and reduce the number of vehicles by one. Do this as long as you can just assign all customers. Then you definitely get better results in terms of number of vehicles employed. But still, there might be problems where you cannot find the best number of vehicles. Here, you probably need to find more appropriate ruin and recreate strategies.