Fernando Alonso's move to Ferrari captured the imagination of the Spanish public as thousands flocked to see their hero make his debut for the team.
A test day will ordinarily attract a few hundred spectators as they are regarded as routine affairs with the teams simply putting a new car through its paces.
But this first week of testing at Valencia's Ricardo Tormo circuit has sparked unprecedented levels of interest, with today proving exceptional.
On Monday, with Michael Schumacher and Felipe Massa returning from retirement and injury respectively, there were 7,200 spectators.
That figured more than doubled yesterday as 15,000 fans turned up, with Massa again at the wheel of his Ferrari, and the 2008 world champion Lewis Hamilton also on track.
But on the final day, with Alonso turning out in Ferrari red for the first time, track organisers are expecting a crowd of 30,000.
By 11am local time, and with traffic on the main highway that leads past the circuit stretching back four miles, over 20,000 had packed their way into the grandstands on the eastern side of the 2.5-mile track.
The 28-year-old from Oviedo did not disappoint either because in his second stint, after only 29 laps in total, he posted the fastest time of the week to date.
Massa, making his comeback after six months out following his horrific crash in qualifying at the Hungarian grand prix, was quickest at the end of the first two days, completing 226 laps overall with his best a 1min 11.722sec yesterday.
Alonso had shaved 0.123sec off that figure after two hours of the session, and was joined by his fellow Spaniard Pedro de la Rosa at the top of the time sheets as Sauber continued to show surprising speed.
Schumacher, this week marking the end of more than three years in retirement, was just over 0.8sec down on Alonso.
However, the seven-times world champion managed to shave half a second off his best time from Monday as he continued to become accustomed to his Mercedes.
Jenson Button, after initial seat-fitting and cockpit issues on his first day in a McLaren, was just over two seconds down, but the third world champion on the track had completed only 14 laps.
The F1 rookies Nico Hulkenberg and Vitaly
Petrov, on their test debuts for Williams and Renault after finishing first and second in GP2 last season, were 2.5sec and three seconds down respectively.