According to Official Guinness Records,
The lowest fuel consumption driving to 48 contiguous US states in a hybrid car is 3.16 litres per 100 km (74.34 US mpg, 89.28 UK mpg) and was achieved by Kyle Morris and Connor Morris (both USA), driving a stock 2006 Honda Insight, between 15 and 27 June 2013.
For a complete list of 2013 records, please visit 2013 Guinness Records in United States.