User blog comment:73.192.181.54/Multiplayer Math/@comment-30074072-20161213131815/@comment-73.170.70.188-20161214104850

From what I have noticed (lacking rigor), tuning kits seem to give stat boosts equivalent to the last/next (can't remember) max upgrade level's change. So if your car has level 4 top speed, the tuning kit will make it act like it has level 5 top speed. I hope you can expand on this if this is provides a new lead, or tell that this is false.