Maybe their own oil companies are keeping it from them as ours are...greatly improved mpg's means alot less profits for the oil business no matter what country and less tax revenues for the government. Just do a search on Mr. Pogues invention and read about the many high mileage patents that have been bought by the car makers. It has and is happening.
But the Soviets didn't have "companies" and did want to surpass us. Ditto for Maoist China.
I've heard variations of this story (100 mile per gallon carburetor, for instance) for 50 years and know the stories go back much farther than that.
Pure fantasy. No company would pass up the money that could be made from selling such a device as well as guaranteeing petroleum remains the sole energy transportation fuel for another century.