According to Official Guinness Records,
The fastest 5m by a rubber band car is 2.82 seconds and was achieved by Marti J. Powell (UK), in Chesterfield, Derbyshire, UK, on 17 July 2015. This record was broken on the Guinness World Record Challengers site (Challengers' tag: martijp): http://challengers.guinnessworldrecords.com/challenges/349-fastest-5m-by-a-rubber-band-car/attempts/6919
For a complete list of 2015 records, please visit 2015 Guinness Records in United Kingdom.