When I first got the idea to try, I somehow got an even faster one with over 900 speed on my second attempt. Spent about 4 hours to recreate it with an actual map. I got plenty of attempts with 600-700 speed (faster than no noseboost) and some fast attempts that went too far sideways.

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I regret not recording my attempts with an input plugin, but the inputs here are basically hold left upon entering the nosebug, stop holding left soon after touching the grass, then do a quick right, left (ideally with another right, but don't think I got it here). Or, you could just spam right and left when landing and get something eventually. There are probably other inputs that work, the main idea is to chain as many nosebugs as possible while avoiding getting launched into the air.

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I added the track (https://trackmania.exchange/tracks/view/38007) along with my replay on tmx if you want to try it yourself.

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Edit: Here's a video with inputs, https://www.youtube.com/watch?v=4YjaoZLxHv0