Your Interstate drifter's series is on my top 3 favorites games at Newgrounds, equally with Tiny Heist and Sanguine 2.
I really like the creativity and originality in the games. There's a lot of details and each environment got a nice style. It contains plenty of diversity which encourage the player to pursue the game. The re-playability is very strong, there's a lot of way in which the player can upgrade his drifting skill (speedrun, take all the coins, try to go everywhere without touching a single coin, etc.).
Regarding the things that could be improved (in my opinion), I would say, for 1998, that D4 : oasis slightly look like E4 : glaze lake. For sure, they both get their own style, but I got the impression that they were more similar in comparison with the other levels. For 1999, I would say that the diablo boss is maybe too easy because it is easier to beat him than to beat the BMW.
So, i think that your interstate drifter's games are excellent. I will for sure play the ultimate 2000 version.
My bad for the grammar, english is not my first language.