Cullum joined ARVAL PHH as vice president of finance in 1982 after 14 years with the Ford Motor Company, which he joined shortly after the successful marketing and development of the Ford Cortina.
He joined ARVAL PHH as the company's 90th employee, when the customer portfolio amounted to 45,000 managed vehicles and 30,000 fuel cards.
The firm now has almost 2000 staff in Swindon, Bristol and Manchester, with responsibility for looking after more than 1.5 million fuel card customers and 386,000 vehicles.
After his retirement, Cullum will retain an active interest in business, continuing as Chairman of the Wiltshire and Swindon Economic Partnership, as a member of the CBI Council and serving as a non-executive board member of various companies, which will include promoting ARVAL PHH.
He said: 'I have thoroughly enjoyed my twenty years with ARVAL PHH. 'It has been a time of dynamic change in the market and in the company, and every day has been full of challenge, interest and reward.'