Nontaxable articles: Chassis manufacturer GVWR & registration weight

Our company is a truck dealership. We frequently sell chassis that are rated 32,900 pounds gross vehicle weight (GVW) by the chassis manufacturer. We don’t charge FET on these chassis. As a service to our customers, we fill out and file the truck registration papers with the department of motor vehicles in our state. The customer signs the registration application. Recently, a customer asked that we indicate on the registration application that the truck has a GVWR of 35,000 pounds He wants to be able to scale more weight. If we do this, does the chassis become taxable?