Over at the Fuel Economy site, users can enter in their own MPG, based on what kind of driving (city/highway) they do.
https://www.fueleconomy.gov/mpg/MPG.do?action=garage

You can still enter and read the site without registering, but you can't add in your own MPG.

Here's a copy and paste of the info from the above site page:

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Welcome to Your MPG!
We've created 'Your MPG' to help you calculate and track your fuel economy and compare it with EPA test ratings. You can also share your MPG with other users.

Benefits of Registering
If you register, you will be able to save your MPG information and view, edit, or update it later. You will also be able to share your MPG with others. Fueleconomy.gov retains no information that could be used to identify any individual with a user name or password.
As a non-registered guest, you still have access to all MPG tools, but you cannot save your data or share your MPG.

We Can Help You...
Other Useful Tools
Our Printable Form for Recording Fuel Purchases
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Obviously it's not for modified autos, just for stock. But if people here have already figured out their MPG, no matter what year or engine or model, they could enter the info over there to help more people.



They even have that handy printable form that should really help tracking your purchases and mileage.



The site is for ANY vehicle (U.S.) going back to 1985. So whatever other cars you have around, go ahead up enter the info there too! That can help when people go to buy used vehicles and want to get a REAL idea of what the mileage is going to be.

 

I have a new Fit. 33.5 with the first tank. 37.5 with the second, average ~35. Not bad with a new engine. I track every tank with the gas to three decimal places, mileage to one and get accurate figures. I measure every fill. I am running about 2MPG below what the dash readout says.