Certain imports were apparently exempt longer. Maybe the exemptions were tied to gas mileage? The Rabbit reportedly returned better than 40 highway mpg, but that's when the national speed limit was 55 mph. I wonder if the smallest Hondas were also exempt. And it looks like some trucks were exempt longer, too. According to this, the largest engines and the largest trucks were exempt, some until 1986: 73-87 Chevy/GMC Pickup Emissions (at http://www.73-87.com/7387info/7387emislevel.htm ) I test-drove an '84 D50 without a cat. Truck rules were apparently different from those for cars. The 1972 report is false. Unleaded was introduced around then, so maybe the author is confusing cats with hard valve seats. And I don't think that cats were mandated; for 1975, a severe reduction in certain pollutants was mandated, and cats were the only technology then which could achieve that. The Whippet article reads better now. In the third paragraph from the end, you might want to add a comma after "dropped".