After fuel prices here in the states declined to under $1/gl in the late 1980's the whole subject disappeared from view about the same time the highway speed limit went back to 65 mph (later to 70). Those little motors wound up on riding lawnmowers. The ones sold here weren't justified by the savings (except when step-daddy bought one for a kid he didn't much like to commute to school long-distance in hopes it wouldn't be seen some dark night), and the whole thing was moot before 1994.
Yes, it has "mattered" since 1970 or '73, but we'd rather watch OJ chased down the highway or debate nonsensical, non-existent political party differences after a few cold ones once the back lot was mowed at 22-mph.
I believe I would search the GOOGLE archive of Popular Science or Popular Mechanix magazine with those search parameters; those two exist off of press releases.
Good luck
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