Complex sentence: Many British automobile companies used undersquare designs through the 1950s, (largely as a result of a motor tax system that taxed cars by their cylinder bore) so many of the most famous cars of that era use this design.
Split into simpler sentences: Many British automobile companies used undersquare designs through the 1950s, largely as a result of a motor tax system that taxed cars by their cylinder bore. Therefore, many of the most famous cars of that era use this design.