I calculate my last 4 fillups. They range between 31.5-32.5 (80% highway)
How I calculate?
When I fill up my gas I reset the odometer an divide by how much gas I have to put in before the gas pump stops pouring ( I always use the same gas station). Anyway I get about the same 32 mpg, so my method of calculation is pretty decent (nothing is perfect).
I drive 25 miles each way. Today (first time using) scanguage II is giving me an average of 25 mpg (90% highway). This must off by atleast 6 mpg. I understand it gets it information from the sensors which is not 100% accurate.
Anyway my main reason for getting the scanguage is to see @ which speed do I get the best MPG on a level highway. Inaccuracy isn't as important as consistancy, hopefully I'll be able to see what consistant speed do I get the best MPG. So far the best steadiest (more than 10 seconds) mpg @ highway speed reading is 27-29 mpg.