7 Gears on any car:

Start a Single Race with either M. Schumacher
or E. Irvine. Make sure that you have selected 
manual transmission. On starting the race, change
gears until you have reached 7th Gear. Then quit
the race and enter Grand Prix mode. Start a new 
season or carry on with another one. Whichever 
car you choose it should have 7 Gears!