This article has reviewed a variety of possible explanations for the difference between
leaded and unleaded gasoline retail margins and the way in which those differences have
changed over time. The most common cost-based arguments for the differences appear to
explain little of the margin differences and are not useful in explaining the changes in the
margin difference over time. Larger average purchases by buyers of leaded gasoline are
unlikely to explain more than a few percent of the average difference in margins. The fact
that buyers of leaded gasoline use credit cards somewhat less frequently also explains very
little of the difference.