Sir Paul McCartney and Nancy Shevell have stepped their romance up a gear with a road trip across America.

The former Beatle and his heiress girlfriend have been fulfilling a lifelong dream travelling the remains of the legendary Route 66 highway in an old Ford Bronco truck.

After starting their road trip from the rocker's home in The Hamptons, New York last Thursday, the couple joined the Route 66 in Illinois on Saturday.

Photo op: Paul McCartney poses with fans outside a petrol station in Springfield, Illinois

The couple plan to follow the route, which was decommissioned in the mid-1980s, all the way to the end at Santa Monica pier in Los Angeles.

As the 66-year-old rocker and his love follow the road, which originally ran from Chicago to LA, they will pass through seven U.S. states, including Arizona and Missouri.

As the pair stopped for a re-fuel in the Illinois state capital of Springfield, Sir Paul was happy to pose with a group of fans for photos.

As Nancy took the photo, a group of young males posed alongside Sir Paul, who predictably put on his trademark thumbs up.

C.J. Lowder, the man wearing sunglasses in the photo, said: 'They looked totally in love.

'They told us they were on a trip and going on to St Louis (Missouri) before heading west.

New love: The couple, pictured together in Antigua in April, are travelling the Route 66 together

'No one could believe it was really him - he was so laid back and ordinary. They seemed a great couple.'

The month-long road trip is believed to be the couple's longest holiday - they previously vacationed together in the Caribbean in April.

A source said: 'The road trip was Nancy's idea as a way of them getting to bond and spend time together - just the two of them.'

Sir Paul and Nancy have been dating since November, but only went public as a couple in May when the singer picked up an honorary doctorate in music from America's prestigious Yale University.

The Beatle divorced second wife Heather Mills in March following a nasty court battle.