importing mazda 5 diesel from europe to us

[nommo]

I was considering purchasing a mazda 5 here in the until I discovered it has a way underpowered engine for a car of its weight. I heard that in Europe they sell a model with a diesel engine that gets an estimated 45 mpg. Now I know we have stricter emissions standards here in the US but it seems that a car that is using 1/2 the fuel (diesel) to cover the same distance would have to produce at least twice the emissions or worse emissions than a non deisel car to make a significant difference from an EPA point of view.
A car that can travel twice the distance on the same tank of gas even if it produces more emissions per gallon is a better bet from an environmental and energy point of view. could a diesel engine be producing twice the harmful emissions per tank of fuel than a non diesel engine.

Any I'm thinking of purchasing in london and importing to us. any thoughts.

much appreciated