By the mid-1960s Dinky had lost its virtual dominance of the toy car market to Matchbox. This is probably why Lone Star decided to drop the 1:50 scale Roadmasters series in favour of a new, smaller-scale range. According to the earliest trade announcements, such as the advertisement which appeared in the trade journal Toys International for January-February 1965, the new range of cars was to be called 'Lone Star Imps'.
By the time the toys reached the public the following year the name had been changed to Impy, but to maintain brand continuity the Roadmaster name was retained in the full title of the series: Roadmaster Impy Super Cars. The average length of Impy models was about three inches, so that they really fitted in half-way between the Matchbox and Dinky-Corgi sizes. Their main selling point was that they had more opening features than the Lesney products, hence the advertising slogan proclaiming Impys to be 'the cars with everything'; in fact, the first edition catalogue lists '13 engineering features' fitted to every model.
(From the book Lonestar, The Toy Company and Its Model Cars, by Andrew Ralston)