ND - How much fuel do you REALLY have? (with numbers and data)

[DirtyHabit]

I have seen a few threads lying around about ‘how much fuel do i have left when the light comes on?’ and comments such as ‘even when its on zero, it just keeps driving!’.

well i found a cool parameter when messing around with my ecu dashboard the other day - ‘Fuel Level’ (%).
What is really cool about this, is that its a Live Feed. meaning its going up and down rapidly on every corner, braking, etc. whereas the needle you get on the Dash is more of a steady-Eddie which slowly drops.

Why this is useful? well, now i know that the fuel sensor is at the back of the fuel tank (fuel goes up when accelerating, rapidly drops when braking. no significant bias turning left or right)
I also now know roughly the landmarks for the Dash lights and Needles positions.

In short - the Dash is reading approx 10% lower at every point, compared to the ‘actual’ level of fuel the ECU is seeing in the tank.

Full Tank: 100% full at the garage. Dash says 100%, ECU reads 91% (its max).
The ECU will continue to see 91% for quite some time, until fuel level drops enough to reach ‘real’ 91%. (i.e, for the first dozen miles you will always see a fixed 91% even on hard driving and sloshing)

Initial Move: when the dash needle finally shows you have used some fuel, is when the ECU’s average is no longer higher than 91%. (roughly)

Half Tank: When the Dash shows 50% (needle at true bottom), the ECU is showing approx 60%

Low Fuel Light - this came on while i was driving so couldnt get a true fuel level due to motion, but its rough average at the time was 18/19% with an eyeball. the needle was just under the quarter. between 20-23% at a guide.

Zero Fuel: this then dropped very quickly in relative terms to the low Fuel Light on the Dash, because when the Dash showed the needle at Zero, i pulled over and let the fuel settle and it was showing 12% left in the tank. (approx 5…5 litres).
Thats a 6% drop in fuel, whereas the needle actually indicates about a 20% drop between Low Fuel and Zero.

Filling the tank: I was a very short drive to the garage forecourt for this test, ECU was still showing 12% remaining once it settled down. A full Tank was then an input of 37.5 litres for the first click.

Hope this data is of interest

below - the current Dashboard im using, showing the fuel level at the bottom. its a shame its a screenshot, as during driving its a very ‘live’ number as i say.

also to note, for anyone wanting to really stretch that fuel distance - when the Dash says Zero, you may have 12%, and you may be able to get another 40+ miles out of it (as its over a gallon). BUT, as the live feed was showing me, the slosh on braking or cornering could very easily get you sucking up air though the fuel pipe. so if you are this low on fuel, just limp it to a garage

[IanH]

Interesting observations, thanks for sharing.

[Chris_Ab]

Yes very interesting. Thanks for that Aaron.

Also of interest is what app you are using for this readout? Quite nice that it has an oil temp reading, which we don’t get in car. Might be something I would invest in.

Via the ODB port I am guessing?

[John_M]

Thanks for this.

It’s a 45 litre / 10 gallon tank. On mine, when the light comes on I can get c. 36 litres - 8 gallons in it. So 9 litres/ 2 gallons left i.e. 20% of full capacity. At this point I give myself 50 miles with confidence, and would be surprised if I couldn’t get another 90.

I have run it until the gauge indicates empty, when there seems to be about 5 litres left i.e. I can get 40l. in it.

I rely on these observations and haven’t run out yet…

The ECU values actually seem consistent with these observations at the low end, for what it’s worth. I assume it is just taking the fuel tank sender input, so it isn’t really adding much. except the interest in watching it fluctuate.

The fuel computer seems quite accurate, so although you’d have to do a calculation, if you really wnat to know what’s been used at any point you can used the trip meter and fuel consumption to work it out, and know what’s left assuming you start with a full tank.

Believe me, I’m nerdy enough to have tried this!

[DirtyHabit]

its the ECUTEK ecu software. there must be 100 different options / readings / graphs i can add to the dashboard. but naturally, a limit to how many are useful while driving.

theres vvt timing, fuel injector cycles, engine load, throttle positions, all sorts.

but for the typical options i have in the dashboard, i think most typical obd2 bluetooth readers would be able to monitor those too. and they would only cost £20.
(someone else may be able to confirm)

[mx5_keeper]

Just to add some more data to the pot I went to fill the car and the range left ticked over to zero miles as I pulled up on the forecourt, I managed to put 38.74 litres in, so would guess a minimum of a gallon, maybe 1.5, left to go.

[germcevoy]

Interesting stuff. Whenever the light comes on the tank takes 9 litres. Roughly 2 gallons at a conservative mpg of 35 means 80 more miles. I’ll never stretch it that far but it’s good to know you can do another 50 without hassle.

[John_M]

@germcevoy

Whenever the light comes on the tank takes 9 litres. Roughly 2.3 gallons

9 litres left = 2 gallons, all but about 3 teaspoonfuls. Are you working in US gallons?

[Wardy5]

I generally try not to run too low, but on one particular run the reserve light came on and I had an indicated 15 miles of range when I pulled onto the forecourt.

On that occasion I managed to refill with 36.8L of fuel, so I still had 8.2L remaining, which would have been at least 70 miles of running! It certainly seemed very pessimistic, but didn’t want to chance it!

These figures seem similar to @mx5_keeper above, if I’d kept going a bit longer

Cheers,
Steve

[RichardFX]

As an OCD type I’ve found this thread interesting. It seems to fit well with my experience in NCs and Mazda3 where the warning light appears with probably 10 litres left in their 50 litre tanks. This suggests anything between 70 and 100 miles to bone dry.

The spreadsheets show I frequently filled up with between 38 and 42 litres. On two occasions only have I pushed it further and then put in about 45 litres - knowing there would be fuel available close by when I got home.

However, I do brim the tank above where the pump nozzle shuts off automatically, because some nozzles are stupidly over-sensitive, especially when filling the NC. So I wonder how much of my ~40 litres is in the car’s filler pipe?