How much cheaper is it to transport goods by car then by a 18 Wheeler?

It isn't. An 18 wheeler can typically haul 22 - 25 tons of freight. A one ton pickup can haul one ton (a typically passenger car, much less). Both vehicles will get between five and eight miles per gallon. However, one 18 wheeler is going to use much less fuel to transport 22 - 25 tons of freight than 22 - 25 pickups are going to use to transport one ton of freight each.