Demand does not cause economic growth. It directs economic growth. An increase in overall demand is only possibly by a preceding increase in the supply of goods and services--in other words, economic growth is what allows society as a whole to demand more than before.
I realize that a lot of people think that, but in the real world, it's not the way things work.
Are you claiming that the only reason more cars were sold in Jan of 2015 is that there was a spike in the production of cars?
In my world, a customer comes in and orders something, and I produce it. Same goes for many industries. The toaster oven producer doesn't just randomly produce as many toasters as possible, expecting that if they increase production, they will magically see an increase in sales. It doesn't work like that. The toaster oven company produces all of the toaster ovens that it's customers (the retailers) order. It gauges it's production based on the number of orders that it gets.
Likewise, McDonalds only produces as many hamburgers as are ordered, and the company that makes those frozen meatlike patties, only produces as many as McDonalds orders.
The barber only cuts as many heads as walks into it's shop. The paperclip company only produces as many paperclips as it's retailers order. The computer manufacturer only makes as many computers as it's retailers order.
In the case of trucks, it's being assumed that the reason that the reason that cars sales has increased, is because consumers have a few more bucks in their pockets, and can afford to purchase more cars. Now most likely the inventory of cars declined a tad in January, because this wasn't really expected, so now car producers who are low on inventory will slightly increase production, until their inventory is back at what they believe to be the optimum (profit maximizing) level. Car sales increased, and an increase in production will now likely follow.
Anyone who is involved with the scheduling of production levels would understand this. Businesses don't operate based upon your theories, they operate based upon really life demand and the need to maximize profits. Having warehouses filled full of production which is beyond demand, rarely if ever maximizes profit.