OBD2 accuracy??

[DaveB729]

I’m new to towing and had learned from some of you all that an OBD is a good investment for monitoring things like tranny temp on hill climbs. We took our new travel trailer on its first trip last week. Had a nasty head wind most of the way and my trip computer (forgot to connect the OBD)showed I got 9.1 mpg. This weekend we took another trip, about the same distance from home, but not nearly as much wind. This time I remembered to connect the OBD and I reset the trip computer. So here’s where things get weird. Trip computer, which everyone says is not reliable, showed I got just over 13 mpg. That seems reasonable. But the OBD, which I thought would be more accurate, says I got 20 mpg! There’s just no way. I filled with premium 91 fuel and aired my truck tires up to 40 psi cold before the trip, but I can’t imagine those things would make that much of a difference. I wish I had filled up my tank as soon as we got back into town and just done the old fashioned math, but had too many other things on my mind.

So, if the OBD is this inaccurate for something as simple as fuel consumption, how reliable is it really for engine and tranny temps? Am I missing something? I’m using an OBDLink MX+ OBD2 with the OBDLink app. I would assume the choice of app wouldn’t matter since it’s just displaying the data relayed through the unit, right. Any thoughts?

Sponsored

 

[ControlNode]

Without knowing how that app was coded I couldn't say. Does the app have a setting for the fuel tank size? Perhaps it tries to use the fuel tank % change from start to finish to determine used fuel? If the tank size is wrong (maybe see too small), or if from a "full" tank it will read 100% for a couple gallons from topped so in theory it used 6.8, but 2 gallons over the 100% were not recorded as used since it only counted from 100% to ending fuel.

 

[OFC Ranger]

I use a wireless reader to a tablet using TorquePro - everything on it is pretty damn accurate but the estimated fuel mileage, which is off by about 1.

I know the fuel level gauge is accurate on it though, if I don't have a full tank the meter will bounce up and down depending if I am accelerating or stopping as the gas sloshes around. lol

 

[BassRanger]

There is no fuel economy PID. The app is doing a calculation.

 

[DaveB729]

There is a setting for fuel tank size and it’s set to 18. On the first screen in the app (not the one in the above photo) I have a display for “fuel remaining” and when I filled up I confirmed it showed 18 gallons remaining. As you can see, app says I burned 4.8 gallons. This seems accurate as my needle was just below 3/4 tank when I got home and a quarter tank is 4.5 gallons. So…the math works out but it still doesn’t make sense. I guess I won’t know until my next trip if I can remember to compute it manually. Thanks all.

 

[DaveB729]

I just thought of something. OfcRanger’s comment about the gauge bouncing when the fuel is sloshing around made me wonder…Would the fuel level sensor show more fuel in the tank than what’s actually there when the trailer is still attached and the back end is sitting lower than normal? It did seem the fuel gauge went down a bit more after I unhitched and dropped the trailer off at storage

 

[notsolinear]

Having worked with CAN data on a lot of off-highway vehicles I would say that it's good to keep in mind that most values you can read with an OBD tool are NOT intended for direct human consumption. If it exists on the vehicle network it's there to do a very specific job, and the intended recipient (usually not you) of that data understands any/all of its quirks.

There's nothing wrong with monitoring values using an OBD tool, but just keep in mind that there could be a mismatch between what you think it's measuring and what it actually is; I would always cross-check with another measurement I trust. For example, an ambient air temperature measurement may come from deep in the intake after a bunch of hot piping so that it never matches actual ambient air temp.

Another big risk, highlighted above, is when someone else's software is making assumptions rather than reporting raw data from the vehicle. Personally I would never trust that fuel economy calculation (apparently) based on change in fuel tank level. The consumption stats from the built-in display are going to be based on far more accurate data from the engine controller compared to the accuracy of the fuel tank level sensor.

 

[ControlNode]

Well, when you filled the tank, how many gallons did it take?

 

[Loweredon33s]

There is a setting for fuel tank size and it’s set to 18. On the first screen in the app (not the one in the above photo) I have a display for “fuel remaining” and when I filled up I confirmed it showed 18 gallons remaining. As you can see, app says I burned 4.8 gallons. This seems accurate as my needle was just below 3/4 tank when I got home and a quarter tank is 4.5 gallons. So…the math works out but it still doesn’t make sense. I guess I won’t know until my next trip if I can remember to compute it manually. Thanks all.

Try 18.8 gallons since that’s the capacity. Might get you better results.

 

[OFC Ranger]

There is no fuel economy PID. The app is doing a calculation.

Yea you set it up after filling the tank to full. It is just for whatever reason not as accurate as the onboard calculator. It is pulling the data from the fuel leveling readings over time.

 

[D Fresh]

How do you know which is accurate without hand calculating?

 

[Dgc333]

How do you know which is accurate without hand calculating?

Hand calculating can be off by 1 to 2 mpg to. If you don't fill the tank to the exact same level between fills that introduces error.

 

[D Fresh]

Hand calculating can be off by 1 to 2 mpg to. If you don't fill the tank to the exact same level between fills that introduces error.

I'll agree with your point, but not the amount.

If you use first click off on the pump they are all generally going turn off at the same level. Even if they fluctuate by 2 inches where they "shutoff," that's not much fuel. If the filter neck is 3" wide, a 2" difference is 21 cubic inches, or not even a tenth of a gallon. The difference would be VERY minor.

Also, this is why when hand calculating the smartest way is to use the same pump.

My point was that the OP is assuming one is right and one is wrong. It's more likely that they're BOTH wrong and his mileage lies somewhere in between the two. In my ever-so-humble opinion.

 

[Nomadjohn]

Having worked with CAN data on a lot of off-highway vehicles I would say that it's good to keep in mind that most values you can read with an OBD tool are NOT intended for direct human consumption. If it exists on the vehicle network it's there to do a very specific job, and the intended recipient (usually not you) of that data understands any/all of its quirks.

There's nothing wrong with monitoring values using an OBD tool, but just keep in mind that there could be a mismatch between what you think it's measuring and what it actually is; I would always cross-check with another measurement I trust. For example, an ambient air temperature measurement may come from deep in the intake after a bunch of hot piping so that it never matches actual ambient air temp.

Another big risk, highlighted above, is when someone else's software is making assumptions rather than reporting raw data from the vehicle. Personally I would never trust that fuel economy calculation (apparently) based on change in fuel tank level. The consumption stats from the built-in display are going to be based on far more accurate data from the engine controller compared to the accuracy of the fuel tank level sensor.

This is so true. Especially for the MPG estimation. Put another way if it were just that easy to get right I bet Ford would have done that from the factory. Here's a pretty interesting wiki page that explains how this works in another app but I'd bet it's similar to the OBD link app. https://wiki.torque-bhp.com/view/MPG

 

[Dgc333]

I'll agree with your point, but not the amount.

If you use first click off on the pump they are all generally going turn off at the same level. Even if they fluctuate by 2 inches where they "shutoff," that's not much fuel. If the filter neck is 3" wide, a 2" difference is 21 cubic inches, or not even a tenth of a gallon. The difference would be VERY minor.

Also, this is why when hand calculating the smartest way is to use the same pump.

My point was that the OP is assuming one is right and one is wrong. It's more likely that they're BOTH wrong and his mileage lies somewhere in between the two. In my ever-so-humble opinion.

Well the 2" fluctuation can be upwards of a gallon or more if the pump kicks-off before the level reach's the filler neck. I typically find I can put in almost a gallon more after the first kick-off and the second kick-off (my 68 Barracuda will take two more gallons after the first kick-off). It varies greatly from one pump to the next, so if you are not making an effort to minimize the variation when you fill the tank yes a 1 or 2 mpg error can easily be seen.

Sponsored