If is singleplayer, how can people join?
Also, as Nonnel, is not working for me the achievement
And I guess you can host a multiplayer server with default speedkart gamemod and make a race by yourself, cant you?
It's not single player.
To get the speedy kart achievement, you have to be the host of a server with at least 2 or more players, and win.
For speedier kart, you need to be on someone else's server with at least 2 or more players and win.
Being last man standing doesn't count.