This could easily be due to your favorite character not being able to keep up. It also will have something to do with the kart body, wheels, and glider that you've chosen - but more on that later.
Thanks to the fine folks over on the Mario Kart 8 wiki, we now have this beautiful character comparison chart. As you can see, the heavier the character, the faster they are, and the lighter the character, the more acceleration they have.
If you find that you can't keep up in the straights, it might be time to switch from Toad and try a race or two out with DK or Bowser.