Believe it or not, the old trucks are "greener" than you might think. The low compression ratios of the older inlines burn cooler than the higher compression ratios of more modern vehicles. As a result of burning cooler, the engines create fewer nitrogen compounds in the exhaust. The nitrogen compounds are primarily responsible for smog. Now I am not saying that a 1953 chevy with a 216 is greener than a modern volkswagon, but it probably puts far fewer smog forming particles in the air than a H2 Hummer.
There is in fact a little bit of a trade off here. Higher compression ratios give more power and better fuel economy. They also form more smog emmissions, which is corrected by the use of catylitic converters.
