6 speeds are pretty much standard now, in European small trucks (vans)
Still you can only get the full 8750 kg CGVW on a mercedes sprinter with the highest axle reduction ratio. With the lowest reduction ratio, these vans will do close to 200 km/h which means that of the new 2006 Sprinter model, there are already a good number available at wreckyards because of rollovers, drivers losing control etcetera...
The lower hp with a 6 speed: Off course there's a reason that semi trucks use 13 speed Fuller trannies instead of a 4 speed
Lots of the weight gain in cars, is added airbags, exhaust catalysers, electric window motors, air conditioning with climate control, etcetera.
euro 4 semi trucks use more fuel than euro 1 trucks because the engines run less efficient to lower the combustion temperature, to lower the emission of NoX....
About my previous statement, and comments on it, i'd have to add that i see that as one of the main reason that GM stopped their 2 stroke Diesels: 2 stroke gives better cards in the game to make a low weight, high performance engine because the pistons work at every crankshaft revolution.
But because 4 stroke technology was more commonspread, they dropped the 2 stroke diesels as the 4 stroke technology was more refined and more knowledge was available from other sources.
I personally, if i was a millionair and only had to work filantropically to serve the world, and not earn an income for myself and possibly a future family, i would have been experimenting with a 4 stroke diesel. well, that would be basically a 2 stroke engine, which uses an extra compression and work stroke, at which water is injected, to extract the last bits of thermal energy in the hot gases by steam power.
Some inventor already made an engine with internal cooling, based on a 4 stroke engine, which became a 6 stroke engine. search Wikipedia for 6 stroke engine