All of those gadgets are wonderfuly useful and all, but one would assume that they'd add an awful lot of extra weight to a car whose (presumed) primary purpose is a racing car. Particularly the jump-jacks, if you accept that their original purpose was to lift the car up for maintenance, something that you'd normally just use jackstands for (which collapse to be easily stored or set aside when not in use)
The Mach 5 has an auto repair function for it's tires right? Putting built in jacks makes sense if you take this into account. Obviously you can't just press the "replace tire" button while the car is sitting on the ground and traditional jacks require the driver to get out of the car. So instead of jacking up the car, climbing back in, pressing the button for the auto repair then climbing back out collapsing and storing the jack and reentering the vehicle you press two buttons and you are ready to go again. I'd judge he extra weight is worth it.
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.
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.
Most likely Speed assumed that the extra weight was from what ever Pops did to counter the pull.
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.
Here's the REAL reason: Back in The Sixties when Speed Racer was made the Mach 5 needed a trunk to be considered a sports car (as opposed to a race car). Under the rules of the Automobile Club de l'Ouest (sanctioning body for the 24 hours of Le Mans) a sports car had to have two seats, a spare tire, and a trunk capable of carrying an officially sized suitcase. Given their prestige most other sanctioning bodies followed suit. Also—and this may be hard to imagine nowadays—in the 50s and 60's sports car racing was largely an amateur sport: most "race" cars were street legal and were actually driven to and from the track. For example James Dean was killed (and his mechanic injured) by an inattentive driver while he was driving his new race car to a race meet in order to break it in. No trunk meant you couldn't carry any parts, tools, or clothing. And yes, history is replete with examples or people arriving at tracks in sports cars loaded like the wagon queen family truckster.