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Fuel calibration?

RuivoM

Active Member
Dec 21, 2019
49
2
Hi, I've been searching about this topic but couldn't find any relative information.

I have the obdeleven and use it on my mk3.5. The app has the real time gauges and one is the fuel quantity. Oddly, it shows quite below the indicator on my dash. For example, when my dash is 1 point above half, I'd expect it to be around 60-70% full, but real time guage quantity on obdeleven shows 43%.

Is it normal?

Thanks!
 

The Daily Meme

Insta: @thatredcupra
Jan 3, 2018
891
448
Cambridge
Have you ever filled up your car, and driven for about 100-150 miles before the first point goes down? then the rest of the points seem to go down much wuicker than the first one? I have... and my cars range is about 450 miles. Which means the fuel indicator is only measureing 2 thirds of the fuel tank realistically.
 

RuivoM

Active Member
Dec 21, 2019
49
2
@The Daily Meme yes I have, but to me it seems the opposite. Obdeleven reports lesser fuel (it says ~40%) when the indicator on the dash is one marker above half. That's what I don't get.
 

The Daily Meme

Insta: @thatredcupra
Jan 3, 2018
891
448
Cambridge
@The Daily Meme yes I have, but to me it seems the opposite. Obdeleven reports lesser fuel (it says ~40%) when the indicator on the dash is one marker above half. That's what I don't get.
Well that is because the OB11 is reading the true value. OBD11 is measureing 50 litres, while the car is only measuring say 40 litres. 40% of 50 litres is probably equivelent to 50% of 40 litres. Which is why it looks like the car is reading a higher value.

Consider this:

Fuel tank at 100% - OBD11 reports 100%, Car reports 100%
Fuel tank at 75% - OBD11 reports 75%, Car reports 100% (due to the calibration error)
Fuel tank at 50% - OBD11 reports 50%, Car Reports 75%
Fuel tank at 25% - OBD11 reports 25%, Car reports 30%
Fuel tank at 10% - OBD11 reports 10%, Car reports 15%

Does that help you understand? OBD11 is reading the correct value, the car fuel scale does not effectively go up to 100%. Imagine you have a ruler that measures 50cm, but the object you need to measure is 75cm long. By the cars logic, until the object it needs to measure is shorter than 50cm, the car will think the object is 50cm long.
 
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RuivoM

Active Member
Dec 21, 2019
49
2
@The Daily Meme nice explanation. Thank you! That makes sense. That's probably because when car indicates 0 fuel (blinking red) it still has a few amount to measure. Still, I don't get why they don't map it correctly for such an easy thing in 2020 IMO.
 

Damo H

Carbon Snob
Staff member
Moderator
Oct 3, 2012
4,083
2,399
Car Length In Front
Hi, I've been searching about this topic but couldn't find any relative information.

I have the obdeleven and use it on my mk3.5. The app has the real time gauges and one is the fuel quantity. Oddly, it shows quite below the indicator on my dash. For example, when my dash is 1 point above half, I'd expect it to be around 60-70% full, but real time guage quantity on obdeleven shows 43%.

Is it normal?

Thanks!
The dots are a waste of time. I use the range on the virtual cockpit and its circular guage around its edge.

In my experience the dots are more like:

8/8 Dots = Full tank
7/8 Dots = 3/4 of a tank
4/8 Dots = 1/4 of at tank
 

Tivver500

Active Member
Nov 23, 2019
135
52
A lot of fuel sensors just measure the actual level of fuel in the tank and the fuel gauge makes a calculation based on that level, together with the physical shape of the tank, to give a reading indicative of the range.... If you picture a fuel tank shaped like a funnel the level drop at the top would be small for a lot of miles whereas towards the bottom the level would reduce quickly.
As with all measurement devices the more accurate you get the higher price you pay!!!
(Written having spent 50 years in instrumentation & control!!!)
 
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