in my humble opinion, its the other way around. if the meter is showing u 100 kmph, u are actually travelling 102.1 km/hr. similarly, all calculations differ. apply the sme 2.1 percent formula to even the fuel consumption. the meter show LESS than the actual reading (both speedometer and per/km indicator). abt the percentage, nopes i only use regular.
to be honest, no i dint. i just took the intermediate reading of the onboard computer (which, as per my calculations, show around 0.6-0.8 km per liter more than tha actual fuel consumption). so when i leave for Islamabad from Pindi, and when i come back, my AVERAGE mileage jumps up around 2 km / liter.....so this is th AVERAGE mileage. so if it jumps from 13 to lets say 14.8 km/liter, that too within a short distance of say 60-80 kilometers, it clearly indicates that the average for that particular trip i m getting is around 17-18. i actually also tried the full tank method once. filled it next to airport, went to islamabad, came back, filled it again, and that day it had given me 17.92 km per liter. so both methods are correct in this case.