Fuelly calculates average MPG by taking the total miles tracked divided by the total gallons filled. So the individual MPG ratings on the tanks don't have any impact on average MPG. In your case, as I write this, your car has 10588.9 total miles tracked, and 289.297 gallons filled.
Sorry about the confusion, looks like your average is a bit lower than some of your individual fuel-ups.
You are absolutely right. It can take a long time to raise that average. My car was averaging 27.8 mpg, which was rounded up from the true average of around 27.76. Over the last 4,000 miles the car has averaged 29.3, raising the true average to 27.83 but it still rounds out to that same figure of 27.8. It's going to take another couple of tanks to finally change that rounded number to 27.9.
The larger the number of tanks keyed in the harder it is to change the overall average as well.
I was thinking about this just the other day. Would it be possible to show a last 3 month average as well as the overall? It would help to see just how the average is trending over the shorter time period.
Sorry, Furrner, got your request confused with the other thread about comparing mileage on vehicles.
We do have the monthly average graph if you have more than three months of data in the system. That should give you a quick visual indicator of how you're trending over the longer term. But I agree it'd be nice to see more precise numbers for different time periods. That's currently in our "to do someday" list.