Best Way to record MPG's

[Weather Spotter]

yep the 90 day is what I use for my stickers on the back of my car and is what I tell people I get.

[rbrowning]

What is that old saying "above all else, to thine own self be true". Any system can be "played" to get favorable readings for bragging rights, but since the whole purpose of this exercise is to reduce your fuel usage, your only fooling yourself with mini fill-ups under favorable conditions.

But the statement that "the averages average out" isn't really correct. You need to take total miles and total gallons to get the correct answer. Consider a simple example:

You decide to take a little trip up a mountain. You go 10 miles and burn 1 gallon of gas, resulting in 10MPG. You come back down the mountain and only burn 1/2 gallon of gas resulting in 20MPG. Average the two and you would expect 15MPG. Right? But if you divide the total 20 miles by the total 1.5 gallons you find you only got 13.33 MPG. One of the rules of statistics is "You can't average averages".

I kept a log of each fill-up and watched the MPG per tank for quick feed back of problems or improvements. But at the bottom of each sheet I totaled the miles for all of he tanks and all of the gallons then recorded a compiled MPG for that sheet which usually covered 10,000 miles or so. Those numbers were very consistent.