Mike,
Your patience level and threshold of acceptance is one of those YMMV issues, but since you asked; I know you will approach this calmly. It's not as though they've been notified and refused to comply. Maybe they're oblivious to the fact. It happens. Seems to afflict owners of black Audi wagons too.
(old joke).
Since the city has an existing ordinance against RVs, maybe you can get clarification if that also includes 40ft trailers. Maybe a photo will help illustrate the exact situation to the civil service person in charge so there's no confusion. If so, you know you have legal standing.
Approach the neighbor you want to remain on good terms with ask about the race car inside; establish some common ground, then courteously mention how long they plan to park the vehicle there. If they say forever or something equivalent, tell them you think it's unsightly and blocks kids and pedestrians from using the sidewalk, there's an ordinance against it, they belong in a storage yard, etc. If they say it's only temporary, mark your calendar days.
They'll probably see your POV and comply. If he/they become defensive, shrug your shoulders and walk away. They are now the bad neighbors, the ones who aren't neighborly and courteous. Give them a few days (your call) to think about it, then call the city. If the good neighbors don't see your POV in this situation, it would have been a matter of time for something else to happen. To show your compassion, I would even apologize to them for having to turn them in. Invite them to the next block party. The result is the same as if you acted like a controlling d!ick. Since you're not and didn't, that's a plus.
Sherwood