Theocoog

I have a question that I am hoping to get some help with. I think my gauges might be lying to me because the fuel economy they report doesn't agree with the numbers coming from the ODB port. My Trip A/B gauges say I'm averaging 18.5 mpg, whereas the ODB port says I am averaging ~15 mpg. Is Ford adding 3 mpg to the real number to make me feel better or what? Does anyone know why this might be?

Here's some background info to back up my claim. I reset Trip A with every tank of gas and my mpg has varied. I've never gotten better than 19.8 average mpg for one tank and that was all highway (65-75 mph) driving. The worst mileage I've gotten from a single tank of gas is 16.5 average mpg and that was mainly stop and go city driving. I have never reset Trip B. It was reset by the dealer during an electrical repair at ~720 miles, so it disagrees from the odometer by that amount. My lifetime average mpg at 3300 miles is 18.5 mpg.

Recently, I purchased an ELM327 ODB-II from Amazon, and I downloaded the Torque app for my phone from the Android Market. At first I started recording all the data I could, but eventually I calmed down and focused on the fuel economy PIDs, and the PIDs for things that I think can influence my mileage.

Back to my original question. I have attached a graph of one of my recent commutes that shows my speed, and the Instant and Average MPGs as reported by the vehicle's ODB port. Here is an explanation of the graph:

• The X-axis is the time. I left home at 35 minutes after the hour and got to work roughly an hour later.
• The primary Y-axis (on the left) shows miles per gallon.
• The secondary Y-axis (on the right) shows miles per hour.
• Data was recorded every second which accounts for some of the noise of the speed graph.

The OBD data tells me that 1) I left home at 36 minutes after the hour, 2) got on the highway 4 minutes later, 3) had good driving for 4-5 minutes, and 4) was in stop and go traffic the rest of the way. This is a typical commute for me and makes up the majority of my driving, that the truck says I am doing at an average of 18.5 average MPGs.

The ODB data also tells me that the Instant MPGs, shown as a grey line, are all over the place. I knew this anyway from watching the green graph (which I no longer pay attention to) go up and down every time I even think of the gas pedal. Finally, and the source of my question, is the red line that depicts my Average MPGs for this trip. Note that the Avergae MPGs starts where it left off the last time I turned the truck off, unlike the Instant MPGs that always start at zero.

So that's it. The ODB port says I'm getting 15 MPG and the Trip A/B gauge says 18.5 MPG. Why the difference?

GatorMedic

Just do it the old fashioned way. Works every time

RDFTS

iTrader: (1)

Yeah even the owners manuals shows to do it the old fashion way. It's probably the most accurate. :-)

Calculating fuel economy

1. Fill the fuel tank completely and record the initial odometer reading
(in miles or kilometers).
2. Each time you fill the tank, record the amount of fuel added (in
gallons or liters).
3. After at least three to five tank fill-ups, fill the fuel tank and record
the current odometer reading.
4. Subtract your initial odometer reading from the current odometer
reading.
5. Follow one of the simple calculations in order to determine fuel
economy:
Calculation 1:

Calculation 2:

Keep a record for at least one month and record the type of driving (city
or highway). This will provide an accurate estimate of the vehicle’s fuel
economy under current driving conditions. Additionally, keeping records
during summer and winter will show how temperature impacts fuel

economy. In general, lower temperatures give lower fuel economy.

Theocoog

Thanks - that took some effort on your part to write that up and I appreciate your time. However, I already understand how to calculate fuel consumption. I guess I didn't phrase my question properly because I am not really interested in which number is more accurate - I know already that from manual calculations that I've done, "the old fashion way", as you call it.

I really want to know why the same vehicle is giving me two different answers. If the Trip A/B and the ODB readings were close - like 1% or less different, I wouldn't have bothered asking, but the ODB is reporting an average 15 mpg and the Trip A/B gauge is reporting an average 18.5 mpg which, according to my math:

(18.5 mpg - 15 mpg)/ 15 mpg = 0.2333
0.2333 x 100 = the Trip A/B is 23 percent higher than the ODB!!!!!

Fuel consumption is a pretty simple calculation after all; distance traveled over fuel used, so you'd think both sets of numbers would be close enough that the difference would be in rounding. I guess the engineers at Ford are using different methods to determine either fuel burned or to calculate the average mpg.

Anyway, I think systems that are reporting on the same thing should agree by more than 23%. Hence my question.

Which is still out there....

Either way, I love my truck!

93Cobra#2771

iTrader: (2)

The trip meter is the more accurate. My scanguage e reads the same obd2 readings, but actually has an adjustment for each fuel tank. Basically, when you fill up it asks you to put in the actual amount of gas you filled with.

One sure way to find out. Hand calculate a single tank and you'll have your answer.

Theocoog

See my previous post please. I am not interested in which is more accurate. I'm looking for an answer to why they have different readings. My own calculations, like yours, show the Trip A/B to to be more accurate, but I think a 23% difference in the readings needs some explanation.

bubbabud

1st the fuel flow sensor is not 100% acurrate it is ball park reading.used for comparison only.2nd the trip A and B readings are calculated useing different time frames. you can set the econemy readout to1of several time intervles and the A. B. measures between resets. I have not reset trip B since truck was new and it shows 20.0 I reset trip every fill up and it will vary by as much 3mpg. I have the econemy set for 30mins and it shows between 19 and 21.2 I have kept a log of all fuel put into truck for 4800 miles and the old fashiond way comes to 20.02mpg. Pretty darn close. note the temperature of the gas when you buy it can make messurable differnce [a gal. of hot gas weighs less than a gal .of cold gas so here in the desert in summer I try to buy gas stored in underground tanks as opposed to gas sitting in the hot sun. Hope I didnt get to far off subject or bore you to tears. Bubbabud 2011 F150 Ecoboost

Theocoog

I also reset trip A with every tank and my mileage only varies if my driving does (city vs highway), but it's never dropped below what the ODB readout says I get as an average mpg. I never reset Trip B and it's been holding steady at 18.5 for 3300+ miles now and agrees really well with my hand calculations.

It's the difference between the Trip A/B and the ODB that I am trying to understand, but I'll give up saying that over and over now and just watch this thread and see if anyone can explain that difference.

pfbz

My first question would be is the data you are looking at actually Ford PID data or Torque application calculated/computed data?

I know the number of actual PID's that come across a generic OBDII interface is pretty limited, and I don't recall ever seeing a fuel economy PID.

So my guess is that the Torque application itself is collecting data and then calculating the fuel mileage meta-data. And who knows how they are doing it... Very possibly they are either collecting insufficient data to accurately calculate the mileage or simply have an error.

Ford, on the other hand, has access direct access to much more data than what is transferred across a generic OBDII interface, and quite likely is using better data to calculate their MPG numbers.

But I'm no expert in OBD data streams, just a wild-*** guess!

Theocoog

THAT is a really good question and could point to the reason for the difference. I will have to investigate and see how Torque is doing the math. Torque is reporting both Average and Instant MPG. I wonder if that data come from the vehicle and if not, what PIDs are being used to calculate it. I don't recall seeing a "fuel economy" PID as part of the standard ODB-II specification either, so the data is derived, but Ford has to do that too unless they are directly measuring how much fuel is being burned. I think the difference has to be explained by either the PIDs that are being used to derive the data or the sampling interval. Or both.

I have control of the data sampling rate, which I've kept at 1 second intervals in the interest of saving space on my Droid, but I bet that since Ford uses a running average and doesn't store long term data so they can probably sample at much smaller intervals. I will try a 0.1 sec interval and see if that makes a difference.

Thanks!