Skipped a few money pick-ups, but I still have $190 to spare when I get Mega Engine. There doesn't seem to be that many slow choices to skip now.
I have no idea why there are two invisible nitros in track 24. I happened to be viewing the CPU's nitro count when I saw it climb to 1 when I thought I picked up all the nitros. Sure enough, when I drive over that spot, I get a nitro. The location is after the sequential pair of zippers, the first ones you come across after starting the race, the ones right before the first time you fly past the crossover. As you hug the far wall, the same side as the zippers you will pick up two mystery nitros, the second one being near that dirt patch ahead.
If a CPU car drives over rough terrain, and no one is around to see it, does it slow?