I have fuel records going back to 19 Dec 06 and only ONE missed record. I couldn't find a way to account for this missing record. If you skip it you end up with crazy gas mileage on the next entry. A way to add a missing record would be nice.... also csv import (but you already said that might be on its way soon).
I have a similar problem - I'm missing one fuel-up - I manually override the formulas in my spreadsheet to give a linear interpolation (the midpoint between the two fillups which are recorded) for that one entry.
I don't think there is an accurate way to compensate for missing data. I think the site policy should be to encourage users to throw out data for fill ups that have incorrect or missing data. There should also be a way to flag bad user data (not the users) for review/removal. This may sound trivial, but if you look at users with my car, you can see one with 17 fill ups and an average mpg of 35+. A 1993 Subaru Legacy is only a 25+ mpg car if you drive carefully and it is probably impossible to achieve 30 mpg. Someone not familiar with this car and/or site might think 35+ is possible and make a bad purchase decision. This is just an example, but highlights why using good data is important.