BRUSSELS, March 31 (Reuters) - Car services groups on Friday warned a stalled EU proposal to ensure fair access to valuable vehicle data could pave the way for unfair competition from U.S. and Chinese tech companies. The vehicle data tussle between carmakers and a coalition of insurers, leasing companies and repair shops underscores the huge potential of the connected car market, which consultancy Fortune Business Insights says could grow to 250 billion-400 billion euros ($272 billion-$435 billion) by 2030. Carmakers, which own the data in their vehicles covering everything from driving habits to fuel consumption and tyre wear, are themselves...