There are a lot of misconceptions about this floating around. There absolutely is no hard cap on top speed at 150 as many people, including myself, have gone faster. There may however be a soft cap at 150 which means that no vehicle can go faster than that under normal -- straight line, asphalt -- conditions. I don't know if anyone has confirmed this for a certainty but it's definitely something that fits in with my personal experience that even taking an Adder, Carbonizzare or Entity out to the freeway and driving for miles on end won't bring them up any higher than the general 130-150 area. There's another misconception related to this, though, namely that "top speed doesn't matter" because of this soft cap as "any vehicle can reach 150". This is absolutely untrue. If the Adder is soft capped at 150, the Entity may be at 140 and the Voltic much below either at say, 125. Cars still differ in their top speeds and by enough for it to be significant. A great deal time spent racing informs me of this and anyone can test it for themselves.
This is simply not true. On social club it says my top speed is with the Entity XF at 151.73 mph. In another thread about this, someone said they got their Akuma up to about 156.
I've gone 165 mph with my Akuma. I believe I know when: during a race which sloped heavily downwards, allowing me to reach very high speeds impossible on a straight road. This along with slipstreaming in races are the ways I imagine most people have reached speeds over 150.
Let him be king over charred bones and cooked meat. Let him be the king of ashes