yeah the computer forgets to factor in when you park your car and its cooking under the sun while the gasoline vapors escapes, lowering your gas mileage. Some systems have a vapor recovery system where some of can be used instead of wasting it all into the atmosphere.

so hand calculation is what I look forward to, just get a recept and write the number of miles on it. Then enter it in the databases when one gets home.

i got around 20 km a liter when i drove a civic 5 spd using advanced hypermiler techniques
and now I drive a salvage nwh20 prius that gets almost to 25 km/l with 195,000 km to date.