Why if a car stat says speed 9,5 and I try the same car with a different tuning which says speed 9,3 the one of 9,3 is faster??
Those stats are more about the potential of the components than a direct indication of speed etc. You can add a Race wing which will lower the Speed stat, but then change the areo tuning to the lowest setting to allow top speed with little drag. The setup with the higher Speed stat could also have a gearing setup for acceleration rather than speed.
It also has to do with the transmission / gear ratios. Sometimes you can engine swap and put the racing transmission in and the speed stat will drop dramatically - like go from an 8-9 to a 4-5. You can then check the gear ratio and they’re all messed up. You can get the speed back up to a 9+ range, but it’ll still show low numbers for speed since tuning doesn’t effect the actual specs.
There are some ways to upgrade cars and keep them in a lower class. Things like that will lower the overall points of the car, but you can just tune your way out of that hole.
This is exactly why Horizon 1 was kind of hard to play sometimes - you could engine swap / transmission swap, but then it didn’t let you tune the car, so you’d be stuck with the terrible gearing ratio