Accurate milage & miles per gallon question....

[Shifu]

The speedo is a concern since it can affect the wallet and our demerit points. The "speedo healer" will get it more accurate if you use the good reference for a given conditions but only for a while. The main reason is that our current speedos get affected by many factors that will introduce an errors, like your tire pressure changes, tire wear, different brands of tire, your lean lean angle, even the tyre temperature not to mention the changing of sprocket ratios which is the most obvious, and finally the wheel slip that varies all the time depending on your speed and your riding habits. Wheelies and wheel sliding out of corners are obvious but the wheel slip occurs all the time as a result of interaction with the road even without any perceptible signs of it. In short, even with the best efforts to get it right its accuracy of mechanical/electronic sensing systems will be limited.
If you want a consistant and reliable readings you need to use a GPS based system as it is independent of all these factors that affect your "normal" speedo. It relays purely on a position (distance) change in a set time which, epending on the chipset used, it will be 1 or 0.5 second. That is the only trully reliable and accurate method and it really should be used for that purpose rather than all the electronic sensors and gizmos they using now.
The odometer on another hand is only meant to be a guide for your service intervals really and if you get it within 5% tolerance band (wheel slip again), I think you would be doing very well and for all practical purposes it is good enough.