We're not going to sugarcoat it- if you're looking for Wii U racing games, your options are really limited. It's very clear that the best racing game for Wii U is Mario Kart 8, and what a great game it is. With the DLC available you can even race as Link for the first time, something that has never been done in a Mario Kart game. Besides kart, some other good Wii U car racing games include Need for Speed: Most Wanted U, and Sonic & All-Stars Racing Transformed. People have said good things about Fast Racing Neo as well, so check out the trailer and read an extended review for that one.
Besides the current gen games, the Wii U store offers some classic Nintendo racing games for download, including Mario Kart 64 and Mario Kart DS. These are fantastic titles, so if you never played them years ago, do yourself a favor and check these out.
Fast Racing Neo
Race the Sun
Rock 'N Racing Off Road
F1 Race Stars