Your symptoms add up to oil on the exhaust, which is far enough back that it only burns when going that fast heats the exhaust far enough.

It is real possible that as someone else stated you have a dragging brake, as they smell like clutches. It is very unlikely that the smell comes from transmission clutches. The trans is a contained unit and one would have to get real close to it to smell it.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician