Trip Computer MPG Errors
 

So it appears to be pretty well known that the trip computer MPG average is optimistic. My first time checking it actual was around 21.5, computer said 24.5. That's kind of a big difference.

Everyone else also gets really wacky readings from it. But my question is, why is it wrong? It should be able to be completely accurate. It's a really simple calculation. How many miles did you drive, how much fuel did it take to get there.

The car knows the mileage really accurately (at least I would hope since it shows it right there on the dash), and it should know how much fuel it's pumping in (I would hope, since it's doing really accurate AFR calculations). So if it knows the fuel really accurately (it has to in order to run) and it knows the mileage really accurately (it obviously does), why the fuck doesn't it know the MPG accurately?

Because Racecar.

I know it's posted somewhere, but we know factually as we know gallons/miles when we fill it up. Our MS3 is not capable of measuring the gallons as they pass through, o I guess that is why it is off. It can calculate fuel used I think, and it guesses based off that.

OR, it uses a flux capacitor connected to a Mister Fusion that can process the data.

I saw fuck it. Leave it on clock and bust out the calculator app when you fill it up. And be fucking glad when you hit 0 miles left of gas according to the dsplay, you really have a gallon or so in the tank.

Actually, you have around 2 gallons of reserve, according to a few sources. But I wouldn't advise driving more than 20 miles after the last line is gone.

I lol'd when my computer told me I was getting 25mpg in the city on the days after I installed my DP/FP/CBE. I don't even use that screen for MPG calculations, I just leave it for sirius track info and do my own mpg calculations.

Obviously it knows how much fuel it injects each time it injects it, because it's monitoring the AFR and injecting the correct amount of fuel to maintain that.

Seems kinda silly that it can't just add up all those little injections.

Yesterday I got my display down to 1 mile left for gas. When i filled up it showed (I think) 13.6 gallons till the pump cut off. So yeah I'd say 2 gallons of reserve is about right if you consider 15.9 gallon tank...minus a bit because the pump cuts off before the tank gets completely full. Though my asshole was still puckering at the thought of having to coast across 3 lanes of traffic to get in the gas station lol.

ftfy

Think about this... how low does your MPG readout get when you're accelerating or going through stop and go traffic? How often do you do that in your car? Your ACTUAL may be 21.5 at fillup and you may be reading 24.5 a lot when driving but just remember that your car is calculating MPG on a second to second basis, not as an average (unless the pu's are different.) So everytime you step on it, you hurt your average, so at the end of your tank after getting a steady 24-25 according to the car at that given moment, you still only averaged 21.5 for the whole tank, all driving habits/situations included.

My Focus MPG readout is an average, and the number becomes more static the further you drive. My first few tanks I've averaged about 26-27 mpg, which will go up as the car gets about 2-3,000 miles. Glad that I've been in reality getting a slight bit more MPG than the car's readout gives me. That's awesome.

I'm talking about the average MPG. The PU can show both average and instantaneous.

Anybody notice the huge difference between the current MPG readout on the dash vs the AP"s current MPG....they're not even close. Cars readout is much more favourable. 1st gen here. And well we are talking about it, what's the diff between Actual AFR and Equivalent Ratio on the AP?

the genpu gives an average, it'll calculate the gas consumption every minute or so (thats what it seems like) until you reset it, but there is another screen that gives you the current MPG

So, the trip computer calculates an "instantaneous MPG" every second or so, and then does a time-average (or unweighted average, same thing in this case) of all those measurements to get the "average MPG"??? That's messed up. That's just bad math. That would be called "average instantaneous MPG" or something, which doesn't mean anything.

And, you're saying that the instantaneous MPG isn't even right? This is bizarre!

I have tracked my MPG using the fill-up method on almost every tank since I got my MS3 three years ago, and I started recording the computer's "avg. MPG" for about a year as well, just to do this comparison. Without doing any statistics, I think the computer is usually about 2-3 MPG more optimistic than the pump.

My wife drives a 2010 CX-9. Its computer-derived "avg. MPG" is dead-on. How can these two be so different? Same mfr., gotta be same computer or at least same algorithms, similar sensors, same engineers doin' the same math. Only diff I see is the MS3's turbo. Can a turbo really screw up MPG readings?

Seems like to me that it calculates your average mpg atm not throughout ur whole gas tank. If I drive on highway for 10 miles it goes up by like 5 mpg

My 08 didn't seem to read correct but I think it did, I usually read a 21-23 mpg average reading on the trip computer and usually saw high 20s mpg when cruising according to the instantaneous readout. Highway cruising it showed what seemed to be correct for the car... 25-26 at 80mph. It seemed correct for me but I never bothered to do the math myself.

I feel like I usually put about 11 gallons in for every 230-50mi driven... ~21-23 so I guess the average was right but not the instantaneous (even though it probably was right, people just find it weird to see such high numbers when cruising although the average is going to account for all WOT runs, or any kind of acceleration to get up to cruising speed and incorporate that into the average.)

If the Avg. MPG was a partial-tank avg., then I would expect it to be sometimes high, sometimes low, depending on how I was driving before I got to the gas station. I would expect the computer's estimate to be centered on my calculator-based MPG (unbiased), but with a large variance. But, the computer's consistently high. I don't think I've even had one tank where the computer had a lower Avg. MPG than I calculated by hand.

When I get home tonight, I'll pull up my log and post some stats...

I calculated my MPG vs computer and yes it was off. But it was closer in the L/100km, in the metric mode it's only 1l per 100km than the screen.
On the hwy cruzing at 120km/h (about 75mph) I get 9l per 100km and on the screen it sais 8l per 100km. The road is flat and no traffic.

But I don't give a shit about good mpg, I wanted to test the computer mpg and l/100km.
My wife's Mazda2 get the same millage than my MS3 so it can't be that bad!

By the way.... fuck hybrids!

Glad my GDI Focus can still get 30+ mpg, once it hits peak mpg closer to 40, w00t w00t, but I haz no power :(

So far I have found the speed to be a happy medium between my '03 Lightning (8mpg) and my '12 Focus (33mpg). Have the fun factor and power with middle of the road mileage (23mpg).

I think my problem will be my right foot. Only have 300 miles on the speed so far... hoping as it breaks in the mileage will be a little better... I also need to remember that it has a 5th and 6th gear...

I averaged about 23mpg per tank as long as I had mine. Like you said though, depends on your right foot. I was getting EPA rated highway mileage and over EPA rated city mileage consistently. That said... the sub-30 highway mileage was less than ideal, but it's a great car and a couple extra bucks a tank made up for it then.

The distance to empty after a fill up is a joke too. Mine shows like 450 miles to empty after I fill it up....

I just bought a 2011, and I hand calculated 25.5 mpg (almost all highway) on my first tank, and the computer said I was averaging 28.4 mpg. I was shocked by how off it was, I wish I would have checked my GTI's actual mpg, because the same driving I "averaged" 30 mpg on the computer

I have yet to hit the 360 magical miles my Trip Computer shows after I fill up...

OK. So, here's some data. 68 tankfuls of gas over 14 months. Most of it is Shell 93 octane. Average speed across all tanks was 30 +/- 5 MPH (lots of rush-hour traffic, FTL!).

Hand calculated MPG averaged over all tanks is 21.0 +/- 1.7 MPG. Trip computer averaged over same tanks is 23.4 +/- 1.7 MPG. Avg. of the difference b/w manual calculation and trip computer is 2.5 +/- 0.8 MPG. In other words, the two are tracking up and down pretty tightly together the trip computer is just 2.5 MPG more optimistic.

The computer's MPG offset may be speed dependent (there was a weak trend), so it may be safer to say that the MPG offset is 12% (+/- 4%) of the average MPH. All but one of my tanks averaged 20-40 MPH, though, so I don't know if the trend extends up to higher speeds. One tank was at 63 mph where 12% would have been 7.6 MPG, but the trip computer overestimated fuel economy by only 3.5 MPH.

OK, enough stats for the evening.

I just use Fuelly for my MPG tracking. It's consistently 2-3 MPG lower than the trip computer lol

I hear you. The closest I've ever gotten was 350, and that was on a 5 hour road trip haha. The light turned on at 330 miles, but I was able to stretch it some because I was coasting down the Grapevine

Shit I don't even get that. I get around 260. Maybe that's with my heavy foot and mainly city driving.

Yeah, that was on a long road trip... straight flat ground for 5-6 hours.

I usually hit ~275 miles by the time I have 1 or 2 blocks left on the indicator.

Wow. Are you a statistician? Nice data, with errors and everything.

Just drive the f'n thing and be happy that you didnt have to buy a prius :-D

Last tank I got 26.8 hand calculated. I run probably 60-70% of my miles on a 65 mph highway at 70 mph. I also raced quite a few cars the previous weekend and goofed around a bit in general. I consistently sit between 24 and 28 however. I could probably get 30-32 if I grandpa'd it around, but fuck that, why did I buy the car?

CS SRI+TIP and only use ethanol free 91 octane.

Back on topic however, trip computer said 29.x I think that a contributing factor is also that it seems to sample slowly...like maybe once every 5-10 seconds? So my guess would be that it doesn't see when your in boost as much as when you coast. It is only a hunch though....and there's the fact that IDGAF. I have always hand calculated as it's the only way to really know what you get.

I agree with the comment above, it may be the trip computer only queries data at time intervals that might not record properly when in boost for short periods... though my Mazda 6 was easily about 1-2 miles off per tank so maybe its a Mazda thing.

You must drive like a grandma! The Focus will calculate MTE based on the avg mpg, the 2nd tank of gas I used I averaged over 30mpg with a lot of highway or atleast higher speed consistent driving, and my MTE showed about 390, I definitely didn't get 390 out of it but if the MS3 calculates the same way you must be driving like a grandma or doing a LOT of coasting, lol

that said, I wonder if that's a contributing factor? How many of you coast and let it show 99.9mpg for a few seconds while braking, etc. I wonder if that small spike to 99.9mpg is the reason for the car's average being a few mpg higher?

It`s very simple. The MPG computer is pure bullshit. That`s all we need to know. Forget stats, forget number theory, forget fancy algorithms, forget driving styles... the MPG meter is a lie.

Far from it. But 90% of my driving is on the highway as this is my daily driver. I drive at around 80 (70 limit where I live) and I get some 300-325 miles to a tank. But it always says over 400 when I fill it up and then drops quickly. So dumb, since even on a long trip I have never gotten anymore then 350 or so out of a tank.

That is pretty odd indeed, but sounds about right for what you get from a tank, I'd make about 60-80mi less than that a tank but the gen1s had 12.9 gallons a tank not 15.9.

yeah, the genpu tank also still has at least 3 gallons left in it once the gas light comes on...

plus the fuel gauge is in no way accurate, and neither is the range (yesterday it said 413 miles left with 1 "block" missing from the gauge, i flashed a new map using the AP and afterwards it read 331 miles)

When I fill up I always laugh at the computer thinking I wont be needing fuel for another 530 miles.
The longest I think I've gone was 380.

Try this next time, for a long trip: instead of applying constant light pressure on the throttle, alternate between getting it up to speed (say 75), then let it coast in gear down to 68~70, bring it back up to 75 gently, repeat. I've been doing this for the last 6 - 7 tanks of gas of highway driving, and it appears to boost fuel economy (hand calculated at the pump of course). The only explanation I can come up with for this shit is because of engine braking and the fuel injectors turning off when you let it coast.