With which you 'voided the warranty', 'allgemeine Betriebserlaubnis' and any insurance. Which in case of an accident trashes your life, even if nothing really bad happened (to you).
There’s a difference between “legal” and what most people's insurance contracts say. Most normally-priced collision policies have clauses stating that if you were driving in excess of 130, the insurer reserves the right not to pay for damage you did to your vehicle, even if was otherwise legal for you to be driving over 130. Have fun even getting liability after that.
Modifying your car in a way that would make it fail TÜV (German car inspections, which other posters have noted are far more exacting than even the stricter US states) would be grounds for non-payment no matter what you were doing, as you’ve intentionally made your car non-roadworthy.
It’s also illegal to modify a car in a way that will fail TÜV, which means, in the eyes of the law, you’re doing the same thing as the guy driving a rust heap that he won’t bother bringing in for inspection because he knows it will fail.
Beginner's Guide to Germany: any course of action whose success relies on an inattentive bureaucrat or non-diligent technician is more likely to bite you in the bum than back home, unless “back home” is Singapore.
