Board Game Arena

I am playing on the France track as part of a 1964 championship with heavy rain expansion enabled, and it says "You may only Slipstream if you are in 3rd or 4th gear" in a gray banner above the track image. Why? I don't see anything about gear requirements for slipstream in any of the {basic, advanced, heavy rain} rules pdfs nor in the on-page rules.

It's the text on the event card of the championship

Thanks so much!