Speed Racer/Fridge

The movie

 * Speed can detect the Mach 5 driving stiffly and pulling to the left a little with the defensive modifications made to it, but neither he nor Trixie sense the weight of Spritle and Chim-Chim hiding in the trunk.
 * Because Chim-Chim and Spritle are almost always hiding in the trunk, thus the car is adjusted with their weight.
 * WHY does the Mach 5 need a trunk?!
 * To make room for Spritle and Chim-chim of course.
 * A more mundane explanation is that in the movie-verse the Mach 5 is a production model car and not a racer, as it significantly differs in design to the Mach 4 and Mach 6 which are very specific types of racing cars called T-180s.
 * Not to mention that they use it in a rally race. Rally cars are almost always production models, highly modified to deal with the abuse and to squeeze out every last bit of speed and handling, but otherwise of recognizable make and model that can be found on any highway.
 * For all the impossible (and totally awesome) craziness going on that bit actually does make sense, he did not notice the weight because it was a casual drive with his girlfriend. In a race he would be paying much closer attention to the performance of the car to detect any problems before they cause him to crash.
 * Except Spritle and Chim-Chim were in the back of the car during the cross-country race as well, and during the hairpin turns where their weight would likely have been a pretty big deal.
 * Spritle and Chim-Chim didn't get into the trunk until the second half of the race (since they were home for the first half), and only after Pops and Sparky had tuned the Mach 5 to compensate for the pull Speed reported.