registered vehicles. Of these, 183,171,882 were classified as "Light duty vehicle, short wheel base", while another 50,588,676 were listed as "Light duty vehicle, long wheel base". Another 8,190,286 were classified as vehicles with two axles and six or more tires and 2,469,094 were classified as "Truck, combination". There were 8,454,939 motorcycles also listed along with 764,509 buses. According to cumulative data by the Federal Highway Administration (FHWA) the number of motor vehicles increased steadily from 1960 to 2006, only stagnating once in 1997 and declining from 1990 to 1991. Otherwise the number of motor vehicles during that period rose by an estimated 3.69 million each year since 1960 with the largest annual growth between 1998 and 1999 as well as between 2000 and 2001 when the number of motor vehicles in the United States increased by eight million. Since the study by the FHWA, the number of vehicles has increased by approximately eleven million, one of the largest recorded increases. The largest percentage increase was between the years of 1972 and 1973 when the number of cars increased by 5.88%. There are three main reasons commercial VIO data differs from data from the US government. The first is due to variation when data is reported by states to the US government. States are required to report registrations using form FHWA-561 once per calendar year or fiscal year. Forty six states end their fiscal year on June 30 and four end in March, August or September. This data is due to the FHWA by January 1 of the following year, creating a lag time of about six months and thereby not accounting for half a year of changes. Second, the government's definitions of vehicle classifications change over time. A footnote added to FHWA datafiles states, "...Data for 2007–10 were calculated using a new methodology developed by FHWA. Data for these years are based on new categories and are not comparable to previous years". Third, the government can include vehicles not in use, or double-count vehicles that have been transferred across two states. According to the FHWA Office of Highway Policy Information, "Although many States continue to register specific vehicle types on a calendar year basis, all States use some form of the "staggered" system to register motor vehicles. Registration practices for commercial vehicles differ greatly among States. The FHWA data include all vehicles which have been registered at any time throughout