Yeah, I've done this many times on my other cars (not the HHR yet, since I've only filled it up once..)

You have to account for the gas in the fuel lines up to the engine, and you can usually fit gas in the filler neck too. Heck, in my Lumina I would usually squeeze 2 gallons in after the pump would click off. I could get so much gas in that it would touch the cap, and yes, fill into the charcoal canister... (Yes, I know that's bad...but when you get gas for 50 cents less per gallon, you try to squeeze in as much as you can. Eventually I got the hang of figuring how much I could fit before it overflowed into the canister though...

 

Okay, miles driven is a simple trip meter odometer reading, is probably taken off the driveshaft and assumes correctly inflated tires of the proper size.

Near-instantaneous mpg would be simple (flow meter somewhere in the line with computation of miles driven versus flow). Average mpg is miles traveled over total fuel flow, perhaps a flash memory circuit.

However, miles remaining has to be based on the amount of fuel that remains and that almost certainly is based on a 16.2 gallon fuel tank. Reverse calculation reveals a different value.

This is not the least accurate analog gas gauge I have ever had but it comes really close. It seems to start to be accurate somewhere between 3/4 and 5/8 full. I do not intend to learn the lower limit of the accuracy of the gauge as I make it a practice to buy gas on my way home at the same station every time, because it is the least expensive, on the day the trip meter passes 200 miles. I know I could wait until the trip meter passes 300 miles but I am leery about waiting until 400 miles even though I get just under 30 mpg per tank on average. After 10,700 miles in almost seven months I am getting 29.79 mpg. The people I know who own PT Cruisers hate me.