We keep it simple and just have a "truck account." It serves as an emergency fund, source of funds for truck repairs, place from which the truck payments come, and place into which next-truck savings go.
A portion of the truck account is kept as cash for immediate access. When the account rises above a certain level, cash is moved into less-liquid but higher-interest investments, but only investments where the principle is secure.
With our goal to pay cash for our next truck, the truck account will eventually grow to the price of a new truck plus reserves sufficient to keep the business going through a period of extended down time.
One down-time scenerio the truck fund protects us against is an accident in which the truck is totaled and several months of time off the road to recover from injuries would be required.
Every expediter out there is a split second away from that happening at any time. It's nice to know that if that happens to us, it might knock us off our feet for a while, but it will not knock us out of the business. Preparing for the major disasters, also covers the smaller events like unexpected truck repairs and scheduled maintenance.