Actually, OHV engines tend to shine at low RPM. The extra mass (lifters, rocker arms, etc...) tends to make OHV engines less efficient at controlling the valves as revs rise. Of course, this is strictly a generalization - camshaft, piston and crank design have a lot more to do with where the engine makes it's power.

I suspect that most small-engined imports were equipped with lower gearing to provide more sprightly performance in town and at less-than-interstate speeds. Remember, Sweden didn't have a whole lot of high-speed motorways back then, and much of Europe was criss-crossed by twisty two-lanes. The US didn't become Volvo's #1 market until sometime in the mid 70's, I believe.
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Speed Racer, '83 240 R, '74 164 E, '93 940 OL1 (Manchester, CT)