The credit-card cost explanation of margin differences is also given little support in
Table 1. The NPD data indicate that buyers of unleaded gasoline did in 1984 make greater
use of credit cards, but the difference cannot justify even a half-cent difference in average
margins. If 6% more of the regular unleaded buyers used credit cards (as was the case in
1984 for self-serve purchases), and if such use added five cents per gallon to the retailer's
cost of selling, then this difference in credit card use could explain just a 0.3-cent higher
average margin on unleaded gasoline than on leaded gasoline. In fact, retailers' additional
costs of credit card purchases is less than 5 cents per gallon, as indicated by the 3- or
4-cent cash discount that some stations now offer." In addition, more than three-quarters
of the credit card purchases in every category are made with the station's own credit card,
rather than a bank card. Presumably, the internal cost of processing these charges is lower
than the fee paid on receipts from bank cards. Lastly, the credit-card cost explanation
provides no apparent explanation for the pattern of increasing and then decreasing margin
differences during the 1980s.