What you are missing is that there is space left for over 3 more gallons of fuel than when you pump until the first click.

 

Please, no offense, but I am somewhat skeptical of that, especially when the amount of fuel I put in the tank fits in well with what the gauge says; half a tank takes about five gallons, and so on. You're saying there is room for nearly that much above the full mark?

What I'm missing is why we're determined to pack ten pounds of poop in a five pound sack....

Moon

 

Filling to the top on a stock manual transmission equipped GD3 will allow you to go as far as 150 miles before the needle on the gas gauge reaches the full mark... Any person that doesn't fill to the top and calculate their fuel mileage at the pump has only a very rough idea as to what their true fuel mileage actually is... It's more like 13.4 pounds in a 10.8 bag .... I am only determined to know what my fuel mileage is under different conditions and following different modifications I do to my car.

 

TC, I take your point and do what ya' gotta do.
As I tried my best to point out before, there are enough variables that I remain doubtful that there is an absolutely accurate way to compute mileage...so I, personally, don't worry about it. The computer presumably uses fuel flow to measure actual fuel usage, so perhaps that component is off the table.
Some of the other variables:
-actual accuracy of the odometer under ideal circumstances; is it right when the car is brand new
-actual accuracy with added complications; tire rolling diameter, tread wear, tire pressure, tire changes
-temperature of fuel when pumped
-temperature of fuel when consumed
(both impact the next fillup)
-sensitivity of fuel shutoff when filling
-effect of flooding the purge canister (that fuel gets drawn back in the loop, or it's supposed too if the canister isn't trashed)
-headwinds, tailwinds, the pascal moon and the infield fly rule...

If you're good with all that, then so am I. However, the onboard computer (assuming you haven't lost it to mods) will give you relative mileage without some of the variables.

Moon

 

New member, just bought a 2007 Fit Sport in Phoenix where it lived all its 123,700 mile life free of road salt. Drove it 2500 miles home to upstate New York. Quickly realized that gas pumps click off VERY prematurely filling this fun to drive manual tranny econobox, no matter how they are set. I manage to get about 1.8 to 2 more gallons in before I either run out of patience or settled gas level is visible about 6" down in the filler pipe. With the tiny tank and the mileage I'm getting, filling it full means about 75 more miles between fill-ups - significant on the long trips I take. I never fill any car to the brim if I'm going straight home and parking it - as the fuel warms and expands, it could cause problems. Filling the same each time on this trip, I carefully checked mileage on six tank fulls. Here's the numbers, most segments were at least 400 miles, with cruise control set (except the city stretch). And yes, I calibrated the speedo and odo at 60, 70 and 80 for 10 miles each on deserted western Interstates. With 75% tread remaining Yokohama YK520s, the speedo and odo were almost dead on, less than 0.4 mph off. Seldom went off cruise on this trip. Payload was 510 lbs people and luggage (call me crazy, I weighed everything after getting home):
83mph = 29.8mpg
75mph = 35.8mpg
70mph = 36.3mpg
65mph = 37.8mpg
60mph = 42.4 mpg
city driving (some Xway) for 200 miles around Albuquerque = 39.3mpg

I was impressed with how fun this car was to drive in the mountains NE of Phoenix. I own several Porsches and although this isn't a sports car, it really wants to be thrown into corners and accelerated out after passing the apex. Stirring the gearbox between 4th and 5th with an occasional drop to 3rd to keep the revs up and speed near the limit was great fun. My biggest complaint was the car gets buzzy approaching 90, and it's very tricky in gusty crosswinds with its quick steering ratio. I had some gusts reported on radio of 40-50mph. You have to be on top of your game at all times in those conditions. All in all, I'd give this car an "A" for driving fun, economy, and space utilitization. Feels nice and tight like it's got another 200,000 miles left in it properly maintained. Going to switch to synthetic oil for my first change with best filter money can buy.

 

My car is nearly 5 years old and didn't come with an on board computer. I have a Scan gauge II but use it to monitor ignition timing, intake air temperature, water temperature and voltage. Yes there are many variables that affect fuel mileage figures.. I have been filling cars and motorcycles with return flow to the tank to the top 17 years with only a BMW R1100 GS giving me a problem because of doing so... It would run rich for about 5 seconds and then straighten out and run fine. I would fill it just shy of being to the top.

 

+1 on what KC and LinotheFilipino are saying. Yes, the Fit can fit some extra gas. This is because, rather than being right near the filler like on most cars, the fuel tank is way up under the passenger seat. This is what allows me to continually amaze naysayers with my awesome cargo room. But I digress. If you really "top off", you're filling up the whole tube from the tank to the filler. Thing is, do you know the exact layout of the system for vapor recovery? How can you be sure you're not overwhelming it?
The Fits are good for a range of 350-400 miles anyhow. That's plenty, and it's not worth the risk of damaging the car or the environment to extend it a little.