Large difference between ScanGauge and OBC
 

I'm often seeing a huge difference between the readings on the ScanGauge and those on my car's onboard computer, while I'd expect they get the same information ?

The difference is often up to 2 liters/100 km and that's a lot when readings are in the 4 to 6 L/100 km range.

The ScanGauge is reading the high numbers, but unfortunately, it wasn't too far off the calculated tank average on my latest fillup - far less so than the OBC.

The ScanGauge has been calibrated twice, -5.8 and -4.3% as it was reading too high. Scanrate is set to normal.


What could cause this difference ?

No, the ScanGauge can't measure fuel consumption. It guesstimates it based on the amount of air entering the engine, which it infers from MAP and RPM, or MAF, which it only reads once a second AFAIK.

It's the best estimate available over the OBD-II port, and it's usually good enough unless your car varies the air:fuel ratio. On my lean burning car, I don't even bother using the ScanGauge for mpg.

is the OBC off by a consistent percentage? the sg readings may be related to the dieselness of your vehicle. Also make sure your refueling technique (same pump, same depth/angle of insertion, same number of clicks, etc) is consistent.

The OBC is usually off by 0.1 to 0.3 L/100km, most often reading low.
At least the Scangauge can be calibrated based on actual consumption ;)

Inserting the nozzle is always done the same way.
Since I started using the Scangauge, I've always used the same fuel station, but not always the same pump. There's no incline.
Inserting the fuel nozzle is always done the same way.

Could EGR affect the air/fuel ratio ?

Bingo. Air-fuel ratio is the key here. That's not reported to the OBD port, so it can't accurately calculate the fuel usage. I'm sorry, but it's not likely to ever be quite right with your diesel.

On my gasser, once calibrated, it's very accurate. It's been accurate to within a half gallon total over the last 2 years. One tank might be a half gallon high or low, but that's all from the pump. The next fill balances it out. I trust the Scangauge more than I do the gas pumps.

OBD mpg gauges usually makes an assumption that the air fuel is close to 14.7 to 1, which is reasonably accurate for a modern fuel injected/o2 sensor feedback gasoline car.

On diesels the air is not throttled, thus the air fuel ratio is all over the place, usually very lean, so you cannot expect consistent results.

I would expect that subtracting .2 l/100km from the OBC in your head would be the way to go for accuracy. the on board computer should have a very good idea how much fuel it is injecting, or at least be off by a consistent amount.

I keep hearing that this and that cannot be reported by the OBD port that I do not believe like AFR and EGR. I do not know if those are just not a standard and each manufacturer uses a different command it poll them but I know that my scanner/tuner can read both along with several other things.

If you can have the PCM reprogrammed the OBC can then be calibrated. How to do this is take the Liters used reads 16L and the actual is 15L you are 1L off. So 1L/15L=6.7% you will need to have the Injector Flow Rate (IFR) adjusted.

IFR is measured in time to flow 1 gram of fuel. So the smaller the number the less amount of time the car thinks is needed to flow 1 gram (more flow). The car is currently thinking more fuel is flowing, so we'll raise the IFR numbers by 6.7%. Note that all the fuel trims will suddenly be off by about that 6.7%, unless we also lower the MAF table by that same 6.7%.

I did that on my car and its allot closer to actual MPG.
If you can't have your cars PCM, ECU, VCM or what ever they refer to the cars computer as reprogrammed sorry to get your hopes up but it could help someone.

Bummer. I had hoped the ScanGauge would give a more consistent result than the OBC.

I'll see how it performs after a few more calibrations.
Overall average and total fuel use wasn't very far off the mark on the last (3rd) fill-up.

I'll have to work my way through 150-some fuelings and see what the average deviation really is.


So I would expect, but it isn't.

10+ % deviation is enormous when you're trying to hypermile.
You think you're doing really great ... until you hit the fueling station :o

10%? .2l/100km is 4% of 48.73mpg. my speedo is consistently 3% high, fyi