Paul Goodman, aged 36, of Royston, Herts, was convicted last July of the attempted murder of drug dealer David Sadiku after ferrying gunman Joel Keenan to and from the scene on August 20, 2005.
However, he failed to convince judges his conviction was unsafe.
Giving London Criminal Appeal Court's judgement, Mr Justice Forbes said: "In Goodman's case, it is clear that he was the driver, but nevertheless, the driver who took part in a protracted and determined attempt to injure or kill Mr Sadiku by use of a fireman.
"On count two - attempted murder - that was carried out in broad daylight in a public place and the part playing by Goodman was significant, providing transport and away from the scene.
"However, we have come to the conclusion that the sentence passed was too high."
Keenan, who was sentenced to indefinite imprisonment for public protection and ordered to serve at least 14 years without parole, had his appeal against the length of his sentence dismissed.
A third man, Hasan Mahmud, found guilty of conspiracy to cause GBH with intent, saw his sentence reduced from 12 years to nine.