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I'd concentrate on finding the cause of the reduced coasting. The extra drag on the car would make it feel like it's making less power than expected. The extra load might cause the engine to run richer than usual, creating the extra carbon / soot that you've seen... But I wouldn't read into that too much, wouldn't surprise me if it might be normal, this is just a reason you decided to look where you did.
Might start by jacking up the wheels, and checking for one that takes excessive effort to rotate
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