I expect this will be a very difficult swap in any vehicle that didn't already offer a diesel option. There are petrol cars that come from the factory with reasonable gearing already, though. For instance, my Insight is at 2175rpm @ 100km/h with a 3 cylinder engine, and my Civic is at 1960rpm @ 100km/h with a 4 cylinder engine.
http://i1072.photobucket.com/albums/w366/aperduefit/HCH1%20Gear%20Ratios_zpsez18d9x7.png[/img]