Game: Yellow Turbo is the longest. Also the game: All turbos give the same speed boost.

Like, take two of the same car at max stats, like the DS Automobiles DS E-Tense(D-Class). Then, have one do a Purple Turbo(Nitro Shockwave) and the other do a BOY(Blue, Orange, Yellow) Turbo. There is no better turbo because doing a Purple is the same as doing a BOY. Proof? Both E-Tenses, Purple Turbo and BOY Turbo, have the same max speed. Try this on a straightaway. I only recommend that car because it was the first car I maxed out before and during the name change.

And I know what you're thinking.
"PhOeNiX, wHy ThE dS aUtOmObiLeS?"
Not just because it's the only car in my garage at max stats, but also because it has the longest Blue Turbo and Purple Turbo in my garage. And don't say, "well, ur wrong lmao".
I'll know. The only way that one E-Tense is faster than the other is because one is Overclocked, and the other isn't. Even with a stock car with stock stats. I'll know. The only way I don't know is if I don't own that car. Be honest if one of the cars is Overclocked, and the other isn't, or the cars have different stats, like Car 1 is (04,10,05,08), and Car 2 is (04,10,05,05). Another thing. PLEASE use the same car for both test subjects. Make sure you didn't screw up and one person is driving the D-Class Lambo and the other is driving the S-Class Lambo. PAY ATTENTION TO THE NAME AND CLASS. Those matter. I'm Phoenix Muddriver, and this was my evidence elaboration for how all four Nitro levels in A9L are the same.