Welcome!
When it comes to modifying cars, Sweden has alot of rules! But as long as there are humans doing the inspections, there will always be some wiggle room...
Take mine for example... The law says:
You can´t have taillights nor signal lights painted black
You are not allowed to increase the output by more than 10%
And further on I believe there are crap about changing the dimension of the tires, and I probably fail in some way by having changed the exhaustpipes.
Changing the colour of the car and not register it might be frowned upon as well... The rear number plate? I´m not sure, I´m not a cop =P
Anyway those are all things that might give me trouble when at the rego, but if you find nice guys
"human guys" you might get a free pass on minor things. Not that the cops won´t bug you the next day....
To sum up, luck is a factor when judging how legal your car is.
And I bet the majority of the cars on this forum are "illegal" in some way...
If you are puking out pollution, if you have a tinted windshield, if you have a supercharger coming through your hood in a N/A car you will of course get banned from the roads.
But what cranky inspector doesn´t see, doesn´t hurt cranky inspector....
Also for them ignorence is bliss
And I also want enhance what "endorfinslaven" told you. We do not have good roads! Of course the major highways are somewhat smooth, but your average road won´t be nice to a stiff sports car!