how does the site compute 90 day average?

[CoyoteX]

I finally settled on a better program to put on my phone to track mileage so now I can get the miles driven per tank easily. I have been doing it the lazy way and putting 1 gallon and however many miles per gallon for the miles. But now I can actually put real numbers in.

So if I start putting in real miles will it mangle my average till the old numbers drop off from the 90 day avg?

Is it total 90 day miles/total 90 day gallons = avg or average the mpg numbers of the last 90 days?

btw: the program I got is called auto 3 in 1 and it has a phone and desktop version for when you activesync your windows mobile phone to keep both copies sync'd up.