Just dug into the methodology for figuring lifetime fuel economy for each car. It looks like the database is set up to average together the fuel economy of each tank to determine the lifetime fuel economy of the vehicle. This leads to a bit of error too. This gives equal influence over the lifetime average to each tank, despite the number of miles driven on the tank or the amount of fuel used.
For example: I often take long road trips. I always top off the tank right before I leave, because I don't want to stop on my trip, and gas is usually cheaper at home than at my destinations. If I only drive 100 miles, then top off, the 100 miles could easily be significantly higher or lower than a normal tank. By averaging the MPG of each tank together to get the vehicles lifetime fuel economy, you give equal weight to that short tank.
If you divide the total number of miles entered by the total number of gallons entered, you'll get a much more accurate calculation of lifetime MPG.
My car only has three tanks, and each was driven to near empty before filling, so my numbers are close using both methods. Using the "average of averages" method you have it listed at 49.8 MPG. Dividing the number of miles driven by the number of gallons consumed calculates to 49.9 MPG.
If I wanted to see my vehicle climb to the top of the rankings, I could easily do it, while being 100% accurate with my tanks. The place I like to fill up has a gradual hill, about 4 miles long, leading down to the freeway, and another gas station. I can get over 90 miles per gallon between the station I fill up, and the freeway. If I stopped at that station and topped off the tank from the 4 miles I just drove after every time I actually fill up, every other tank would be 90 MPG. even though this tank was only four miles, it carries the same weight as all my 600 mile "full" tanks.
A lot of people fill up when they find a lower price on gas, regardless of how much gas they have left. The lifetime average could be significantly off for them, or for anyone who fills up with varying amounts of gas left before empty.
Anyone follow that?
