coupe-se said:
Eh???
If the I reset the DIS when I fill up; which I always do; surely whenever I look at the DIS average MPG on readout 2 it will always display the overall mpg UP TO that point?
Thus if I fill up at half a tank the DIS will show the average mpg over that half of the tank!!
Cannot see how brimming a tank alters the mpg reading on the DIS?
OK, two issues here.
Because you are comparing an average (the DIS reading), against an absolute (the actual volume of fuel you squirt into your tank) - you need to a/ understand averages, and b/ realise differences in petrol station pumps.
The average first. An average, by its very nature, improves in accuracy with more data. So, a relatively short run, of say less than half a tank full, could lead to errors in the calculation of the average. The longer the run, the more data is captured and calculated, the more accurate the average will be!
Now, the actual volume of fuel. Different fuel stations can have the pump "cut-off" set at different levels, some even pump at different delivery rates. Then there is the environmental issues (hot vs cold ambient temps) which can affect how full the tank is filled, using the cut-off. Unless you can guarantee that you use the identical pump, on the same filling station, at identical ambient temps, then there will be a discrepancy in the actual fuel volume you replenish in the tank. This discrepancy won't affect the DIS readout, but it will affect the "true" volume of fuel used!
You could even leave DIS2 without a reset for 3 or 4 tankfuls, and providing you brimmed it on the intial re-set, and then again on the final reset (and you keep your fuel receipts) - you should have a very stable and accurate average in your DIS.
HTH, rgds