The deal includes further payments over the next three years, based on targets capped at $33.7m (£23.8m)
United bought the firm as part of its plan to strengthen the automotive arm of NOP World, its US-based research division which is striving to become the market leader in the automotive, healthcare, media measurement and business information sectors.
It faces tough competition from Kantar, the information and consultancy arm of WPP Group, and Taylor Nelson Sofres.
A United spokesman said Allison-Fisher's client list included all the major US car manufacturers except Ford, which is already a client of NOP World.
NOP World is understood to have European ambitions for its new acquisition.
United's shares dipped slightly this morning from 715p to 710p.