What would be the estimated useful life of a second hand vehicles?

For a second hand vehicle, you need to find out how old the vehicle is from the owner at the time of purchase and deduct the number of years the vehicle has been used from the normal useful life. For example if the useful life of a truck is 12 years and it has already been used for 5 years at the time you purchased this truck, then 7 years is the useful life. 

Generally you determine the useful life by the normal operating life of the vehicle(fixed asset), which is defined by the owner.