As previously noted, to get max speed minecarts in Minecraft at the moment through the use of the /gamerule command, you'll need to be playing on Snapshot 24w33a or above and also have cheats enabled.

In the most recent Minecraft Java snapshot, 25w31a, Mojang unveiled a brand-new F3 debug interface and settings. This feature will arrive with the next game drop, releasing sometime in the fall.