Original sentence: What must, then, be considered in evaluating the program is how much pollution is saved by the average participating driver in, rather than driving the clunker with its higher fuel consumption for longer, driving a new, more efficient, car while ammortizing its manufacturing cost (since the new car is being worn down for a number of miles which the clunker could have been driven before replacement)
Edited sentence: What must, then, be considered in evaluating the program is how much pollution is saved by the average participating driver in, rather than driving the clunker with its higher fuel consumption for longer, driving a new, more efficient, car while amortizing its manufacturing cost (since the new car is being worn down for a number of miles which the clunker could have been driven before replacement)