I especially like the Twitter integration to make it as easy as possible to input your data, even while standing at the pump.
Cars: Track your MPG with My Mile Marker
Beta webapp My Mile Marker (M3) tracks your fuel purchases and odometer readings and calculates your gas mileage. Sign up for a (now) free account at the M3 beta, enter your car(s), and their odometer readings. Each time you visit the pump, enter your current odometer reading, the number of gallons purchased and the price per gallon. You can add your data via the web site or via SMS while you're at the gas station with Twitter.
To do so, first add the mymm Twitter user to your list of Twitter friends. Then:Create message like D mymm [miles] [gallons] [price]. For example, a message might look like this this: D mymm 15476 15.34 3.129 The odometer read 15476, and you filled up with 15.34 gallons at $3.129/gallon.
Haven't had a chance to get some real data into M3 yet, but as soon as I get to the gas station...
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