Do you have checks on the reasonability of data entered? Just after entering a fill up for my Civic that calculated to the excellent 6.0 L/100km I noticed a recent fill up for somebody else's Civic that was supposedly 2.7 L/100km (or about 48 mpg!). I assume it was because of a typo.
I imagine it would be difficult to set standards because of the wide range of vehicle types but perhaps a comparison to the historical record of the particular vehicle would work.
The vehicle in question is at
http://www.fuelly.com/driver/barefootvet/civic
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