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I recently purchased a 2007 Honda Civic Hybrid here in Ottawa, Canada. I am still running on the first (dealer-supplied) tank of gas. Earlier today, the Low Fuel Indicator came on with 2 bars left on the Fuel level display. Odometer reading at that time was 717 kms (445.5 miles). Tripmeter A (which is going to stay my lifetime tripmeter for now atleast) says I'm getting 5.4 l/100 kms (43.5 mpg). But somehow the math doesnt add up.

Fuel tank size = 50 litres (approx as per manual)
Fuel remaining when light comes on = 4 litres approx (Manual says 3.5 litres remain between when light comes on and display bars drop to 0 or E. plus there about a half litre in reserve after it hits E)
So Fuel consumed so far = 50 - 4 = 46 litres approx = 12.15 US gallons - (between when it was topped up and today when the low fuel indicator came on)
Based on this I am getting a "calculated" FE of about 6.4 litres / 100 kms (36.75 mpg). But my tripmeter A still says 5.4.

So possible causes ...
1. displayed FE is way way wrong (I've read that the displayed FE can have upto a 10% error but most posts indicate that actual calculated FE is "better". So this scenario would mean that my car's computer / fuel calculation system is wrong.
2. Dealer didn't fully fuel up my car initially. Not sure why they would do this. But this seems to make the most sense. This would explain why the displayed lifetime FE is not aligned with the tank FE.
3. Honda's manual is wrong and the tank sizes indicated are not correct. Again I'd be really surprised if this were the case.

I am not really too concerned about the FE itself (the car is still breaking in and its been cold (around 0 Celcius) and I'm no expert hypermiler). Its just the inconsistency that's bugging me. I'm wondering if anyone else has seen anything like this? I will have my first fillup tonight and i'll be using Tripmeter B to measure tank FEs but I don't think I'll have an answer to my confusion till my next fillup. The only way I can know for sure what FE I am getting is if I compare my own fill ups and that might tell me which of the above causes applies in my case.

Welcome to CleanMPG!

I'd suspect #2

I would go with the dealer not filling the tank to 50 liters. My mpg displayed is always 10% off on the optomistic side. I hear the HCH II is just about dead on. Start keeping track of how much fuel you put in and how far you go and compare the results over several tanks.

Don't worry about the indescrepancy for this tank. There are too many unknown variables for you to get an accurate idea of what your true mpg was.

Well ... quick update ... the cause is pointing towards #3. At the fill up ... with just 1 bar left on the Fuel display and going 35 kms since the Low Fuel Indicator came on ... I was able to fill up 39.637 litres (I stop at first click).

So my calculated FE comes to 5.27 litres / 100 kms a bit better than the displayed FE of 5.4.

So now the question is ... is Honda lying about the size of the fuel tank? Or about the amount of fuel remaining in the tank towards the end? Based on this fill up ... there's about a 10 litre capacity that appears to have disappeared. I'm guessing the "first click" takes up upto a litre ... so there's still about 9 litres ... and that seems a bit much to be the reserve fuel capacity.

And to complicate things ... on some thread on this forum I read about some guy who actually drove into a gas station with a totally empty tank and filled up 13.22 gallons which equals 50 litres. So needless to say .. I'm confused :)

Well atleast my FE isn't too bad.

The filler tube can add a lot if you top off your tank. Also, the HCH I has a 13 gallon tank It could be he had one of those.

On my first fill-up, I drove for 45k's after the last bar disappeared and still I managed only 40 lit. to the first click. I haven't had the opertunity to push the milage since, before fill-up.

And to complicate things ... on some thread on this forum I read about some guy who actually drove into a gas station with a totally empty tank and filled up 13.22 gallons which equals 50 litres. So needless to say .. I'm confused

Hehe, that was Tiger. That still baffles me too.
http://www.cleanmpg.com/forums/showthread.php?t=666

It takes some heart to try that stunt. Even knowing that I could get that far ... I could never try it.

I need to take back some of my comments ... the fuel tank size ... as per my manual is 46.6 litres (12.3 US gallons). I had downloaded an online copy that listed it as 50 litres (13.2 US gallons) which i was going by. In future I think I should just RTFM :)

Hi All:

___If anyone in the Chicago area would like to loan me their HCH-II that is already on fumes, I would be glad to find out how much the tank actually holds ;)

___HCHOttawa, don’t worry about an exact fill on the first fill-up. The FCD is pretty darn close so let that be your guide and the actual miles driven over the fuel amount purchased your actual since that is really your actual. I wish we would have met sooner. The first thing I do with a new car is hit the station given who knows what the dealership has placed in her. F on the gauge could mean just 7/8 of a tank? The dealer fill up is more then likely not even close to full so you know that is not in your favor. On the plus side, Honda tanks usually have a lot more cap then stated in the specs when topped off. The PZEV Accord’s 17.2 can hold ~ 19.7 gallons from empty, the MDX’s 19.2 can hold south of 21 and the Insight 10.6 could hold just a touch under 14 … I also know the non-PZEV HCH-I’s can hold 16.1 from MT to a top off vs. their spec rated 13.2 gallon tanks as well. Never however use their stated cap or actual cap for your fuel calc’s. Use your actual miles driven over actual gallons filled.

___Just wait until temps come up and you get some miles on her with the tires pressed up. On those empty back roads where you have total control, she is going to show you Tarabell numbers and it will be a jaw dropper once you see and experience it for yourself :)

___Good Luck

___Wayne