FYI, of the 8 tanks of fuel I put through
my 2000 Insight where I recorded both the calculated and indicated fuel consumption:
+ 2%
+10%
- 3%
+ 11%
- 2%
+ 2%
+ 1%
+ 1%
Where a positive value meant the onboard gauge showed a higher MPG reading than calculated.
Average: +3% optimistic.