I came up with a solution. It's not pretty, but it works.
It starts with a field common to all records called Fuel Difference. It calculates how much more or less fuel the current person adds in their record than the last person did in their record.
The next step reveals the limitations of this method. A calculation field we'll call Bob's Fuel Bank checks to see if Bob is the driver listed in that record. If he is, it reads the current Fuel Difference value; if not, it enters zero.
Next is a Summary field called Bob's Total Fuel Bank which calculates a Running Total of Bob's Fuel Bank, with Summarized repetitions set to Individually.
Then, Bob's Total Fuel Bank Adjusted subtracts the current Fuel Difference from Bob's Total Fuel Bank so the current trip's lack of data doesn't throw off the numbers - since this data will be viewed at the beginning of the trip before the driver has added any fuel.
These 3 fields are duplicated for each driver involved.
Finally, Driver Fuel Bank, a field common to all records, shows the appropriate Total Fuel Bank Adjusted based on which driver is listed on that record.
Of course, the limitations to this method are that you have to create calculation fields for each driver involved. But, in this case it's a small, static list; so that's not too much of an issue.
Any suggestions as to how to make this process better are appreciated.