Like most here, I've noticed my average fuel economy reading is optimistic, if this can get it closer to the real world it'll be great. I wonder what effect it will have on the miles to next fill up choice? I guess I'll find out.

 

I have experimented with the settings on my OB. It was reading 2-3 mpg optimistic compared to manual calculation. I first reduced the setting to -5 and that made the reading a bit pessimistic. I changed it to -4 and now it is very close to actual calculated mpg. Within a few tenths of a mpg. Winter fuel and weather has knocked down actual mileage from close to 30 mpg to about 26.4. I'll keep monitoring it and see what happens when the fuel changes back in the Spring.

 

When you say that it's within a few tenths, are you forgetting it only changes in 0.3 increments? That alone can account for as much as a 0.3 MPG difference.

Unless there's a way to change how precise it is I'm not doing anything like this, the accuracy on mine varies based on the type of driving. I took a highway trip, the readout was only .25 off, if that, but my normal driving sees as much as 1 MPG off.

 

Now this is the good stuff I like finding in these forums!

 

Thanks! Gonna try to figure out my average difference on MPG and see how well it works.

 

Pretty fortunate here, I guess. MPG and temp readings are pretty much right on with our car "right off the shelf." Clock is accurate too.

 

The accuracy may have something to do with the winter/summer fuel blends too..Just 0.02 cts here

 

Does anyone know EXACTLY how the car measures mpg, or more concisely, fuel usage?
Does it use injector pulses and approximate fuel usage per pulse?
Does it have some way of measuring fuel as it is passes through the fuel lines?
Does it use a set standard for the amount of fuel that should be used per rpm?

It obviously uses the speedometer to figure the mileage part, which could account for quite a large variance, given no car has a 100% accurate speedo.

Just curious...

 

I would reckon most manufacturers calibrate the odometer and speedometer to be optimistic to read distances greater and speeds higher than actual.
This creates customer satisfaction that MPG is higher than reality. It also helps create an illusion of better performance. It also helps to keep the traffic cop away.
How Subaru calibrates the PCM is probably proprietary.

 

This should be a sticky - very good information

Jim

 

These gauges work off the ECU, by measuring the duty cycle of the injectors. Injectors can have small variations in fueling rate which end up having an observable impact on accuracy. But I think that the accuracy drops when the car isn't moving. I'm pretty sure my readings are better for 100% highway than for stop and go.

 

Thanks mxfrank, kinda what I thought. And yes I agree. When I take long trips with few stops my mpg is usually very very close to actual. But my wife drives the car daily with a very short commute, it can range as fars a 2mpg off.

 

Saw this article with a response from a GM engineer.

Your Fuel Economy Gauge Is Fibbing - Edmunds.com

For what it's worth, I have a 2011 3.6 and my gauge has been over by 1 mpg averaged out over a year. It varies based on type of driving and by the tank but hasn't really varied summer to winter or vice versa.

 

Saw this article with a response from a GM engineer.

Your Fuel Economy Gauge Is Fibbing - Edmunds.com

Am I missing something here? In the article Clark of GM blames the optimistic gauge on ethanol. Steve Manzor blames it on dirty injectors. Wouldn't both of these cause the guage to read lower rather than higher?

 

But that lack of energy content would be reflected in the less distance traveled, at least according to how the article states the trip computers work.

All else being equal, if ethanol-laden fuel is only 90% as effective as good-old gasoline, a car will only travel 90% as far on ethanol gas as it would if it were using real gas. So although the volume of liquid passing through is the same, the difference in distance would result in the trip computer already factoring in the difference between ethanol and straight gas.

So, in other words, the manufacturer's excuses for why the comptuers are inaccurate are complete hogwash.

 

All else being equal, if ethanol-laden fuel is only 90% as effective as good-old gasoline, a car will only travel 90% as far on ethanol gas as it would if it were using real gas. So although the volume of liquid passing through is the same, the difference in distance would result in the trip computer already factoring in the difference between ethanol and straight gas.

So, in other words, the manufacturer's excuses for why the comptuers are inaccurate are complete hogwash.

With ethanol, same volume of liquid resulting in less distance traveled would equal lower mpg on the display as compared to straight gas.
I don't see where the trip computer is factoring in any diffreence.

 

Can't get it to work....

I tried the instructions to adjust my MPG for my 2011 Outback, and either I am messing up a step, or does not work for all vehicles ?? All that I got was my Trip Meter back to 0...... Can anyone help me figure out if I am messing up a step or what ?? Thx....

 

I tried the instructions to adjust my MPG for my 2011 Outback, and either I am messing up a step, or does not work for all vehicles ?? All that I got was my Trip Meter back to 0...... Can anyone help me figure out if I am messing up a step or what ?? Thx....

I had the same issue the first couple of times I tried. When you are going through the lights on/lights off stage, you need press the reset button quickly, don't hold it down at all. Once the LED menu readout between the tach/speedo shows up, you can then hold the button to change options. I found once I was done my mileage from the last time I reset was still there.

 

You only have 10 seconds to do the procedure that gets you to the change options. It is very likely to take several tries before you get in. I think it took me at least 3 tries.

 

Finally ......

After numerous attempts again, for some reason the electronic brain cooperated and I adjusted my F/E MPG down by -4. Will be a while before I drive enough to fill up again and see if it is closer to reality now, but I do appreciate the info and moral support in trying again. I still have no idea what I did different that it finally cooperated.... I also hope I did not change anything else....

 

This is pretty clever...kind of like up up down down left rright left right b a b a sleect start for contra!!! Thanks for the info

 

Fantastic information, thanks OP! I will definitely do this ASAP, as the indicator is about 8% optimistic on my car.

 

Mine is off about 6% to high, got it adjusted. Thanks

 

Same here, mine was off 5.9% so right in line with yours. Just adjusted it, so thanks for the tip from this link. Really like this site:29:

 

This should be a sticky.

 

...very interesiting read. I still rely on manual calculation to be exact...like everyone here...guage is optimistic...but I will try this adjustment.

 

How the car calculates fuel economy

I've studied this, and found a few articles on it. Cars that calculate fuel economy generally have a mass air flow sensor. The computer can use the input from that sensor in grams/second. Using the stoichiometric ratio of 14.7 parts air to 1 part fuel (assuming that's what the engine's mixture is), it can determine how many grams per second of fuel is being used. That's how the computer knows near exact fuel usage. A huge equation can figure this number into miles per gallon. Older cars (like my 99 Crown Vic) use the archaic method, watching the fuel level sensor go down and figuring it against mileage. That method stinks.

Here is a link to the article that explains this:
http://www.motor.com/magazine/pdfs/122011_05.pdf

 

I've studied this, and found a few articles on it. Cars that calculate fuel economy generally have a mass air flow sensor. The computer can use the input from that sensor in grams/second. Using the stoichiometric ratio of 14.7 parts air to 1 part fuel (assuming that's what the engine's mixture is), it can determine how many grams per second of fuel is being used. That's how the computer knows near exact fuel usage. A huge equation can figure this number into miles per gallon. Older cars (like my 99 Crown Vic) use the archaic method, watching the fuel level sensor go down and figuring it against mileage. That method stinks.

Here is a link to the article that explains this:
http://www.motor.com/magazine/pdfs/122011_05.pdf

The computer knows almost exactly how much fuel it is using as it has to calculate the fuel injector duty cycle. This is based on inputs from a variety of sensors in addition to the MAF and the maps within the ECU.

 

i use whats in my signature to kinda give me an idea better, i always have around 24 mpg avg in the outbacks mpg avg. gauge but this fully site tells me other wise,22... i hit the gas pretty hard 8 out of 10 times ... so if i am getting 22-24 i think its that so bad lol but i dont know how much i am getting accurately from fully when i do say i am driving 60-65 highway, when thats really all i drive on is just highway... beside on base and 1mile and half to the house... this should help me abit more

 

I knew this thread was around here somewhere. At my last fill-up the MPG for that tank was showing 25.2 but my calculated MPG was 24.6.

With them being that close I'm wondering if it's worth trying to adjust the meter in the car.

 

I adjusted mine to -4% and it came out agreeing at the pump twice - 26.4 MPG give ot take 0.1. Driving 50/50 Hway.