MPG reader inaccurate

Recently I've compiled my real mpg (full tank trip miles/gallons) vs. the car's calculated Average MPG for each trip between fill ups and I found that the computer is slightly "optimistic" in my efficiency. Here's my results:

 

Date____ Station Price _Gallons Total __Miles Trip RMPG AMPG Diff

12/31/07 Forbes $3.139 13.908 $43.66 17794 309.2 22.2 23.4 1.2

12/02/07 Forbes $3.159 13.638 $43.08 17485 346.5 25.4 26.7 1.3

11/11/07 Forbes $3.099 13.178 $40.84 17139 305.3 23.2 24.0 0.8

10/28/07 Forbes $2.839 12.462 $35.38 16833 317.9 25.5 26.7 1.2

10/14/07 Forbes $2.739 13.865 $37.98 16515 345.9 24.9 25.8 0.9

09/28/07 711.... $2.859 13.229 $37.85 16169 334.1 25.3 26.1 0.8

08/14/07 Forbes $2.899 13.065 $37.88 15838 ????? ???? ???? ???

 

Bought the Car (4EAT 2.5i) in Sept 07 with about 15500mi on it so the first trip is not calculated since it was mid tank and not my driving. So pretty much the computer is off by 0.8-1.5mpg and always above. This is rather shocking to me since I already thought this car gave poor gas mileage (coming from a fwd econo box) and now my actual mpg is worse! Anyone else notice that the computer's mpg is inaccurate and giving the driver the impression that they are doing better than they actually are? Also could this be the result of faulty programming (something that can be reflashed) or just inaccuracy that Subaru ignored from the start and should possibly be held liable for?

Great work. The onboard MGP calculation are generally 1-2MPG higher than what you get from miles/gallons used. The display is great for getting an good(not great) idea how your doing on gas, but your right it is off by a bit.

Yes, it's usually off by 1-2 mpg.

Here is my take on it. I believe the computed mpg is calculated from the injector pulse. Each pulse equals so much fuel, so calculate the distance travel from the speed of the car or take it from odometer and divide that by the number of injector pulses to get your mpg. This doesn't take into account the amount of fuel lost due to evaporative and other losses. I think other manufacturers just put in a built-in factor to take care that but Subaru didn't. I'm sure there is some disclaimer in the owner's manual saying something like that number is just for reference only. I don't think it's something you can hold Subaru liable for. Just keep in mind that number is off by 1-2 mpg and it's not a big deal.

Yes, it's usually off by 1-2 mpg.

Here is my take on it. I believe the computed mpg is calculated from the injector pulse. Each pulse equals so much fuel, so calculate the distance travel from the speed of the car or take it from odometer and divide that by the number of injector pulses to get your mpg. This doesn't take into account the amount of fuel lost due to evaporative and other losses. I think other manufacturers just put in a built-in factor to take care that but Subaru didn't. I'm sure there is some disclaimer in the owner's manual saying something like that number is just for reference only. I don't think it's something you can hold Subaru liable for. Just keep in mind that number is off by 1-2 mpg and it's not a big deal.

The big deal is that I'm sad how horrible this car's mpg is! lol. Right now I'm showing 21mpg on the display and I'm probably getting 19-20... and I'm driving VERY conservative, gradual accelerations, coasting to stops etc. I guess I'm going to eventually get another fwd econo box for daily driving, and leave the legacy for bad weather and long trips.

Yeah, 2.5i's 4EAT and city driving is not so good for the MPG. I average 26-28 mpg on summer gas and 25-26.5 on winter gas with my LGT 5MT. My commute is mostly highway so it's not so bad.

Doing some more calculations I guess this means that the computer does not account for 0.8-0.4 gallons each fill up which I assume is lost from evaporation.

Doing some more calculations I guess this means that the computer does not account for 0.8-0.4 gallons each fill up which I assume is lost from evaporation.

 

No way is that much gas lost through evaporation.

 

After doing fuel calcs, the mpg display in my car seems to be off by .5-1mpg, which I guess isn't all that bad. I just wish that there were a way to recalibrate it, say for something like a larger fuel pump, bigger injectors, swap etc.

it *might* not fully calculate for idle times or something, i was stuck in light NYC traffic over the holidays and still ended up with 22mpg for that leg....

i just keep feeling that its either not taking into effect fuel used at startup or something with idle, but its something really small that adds up to +/- 1mpg

 

that being said, i got better gas mileage around town (22-23mpg) with my 5mt LGT than i did with the loaner 4eat outback (19-21mpg) on the same commute to school and back, suburban driving, 8mi one way for two days

Mine has been off by about 0.2. Car has about 1200 miles on it so far.

I am surprised by how poor gas mileage you are getting with a N/A. In my turbo 4, I get between 21 and 24. If I baby it, I can get up to 24 - and I NEVER drive in 'intelligent' mode. When I have some fun, like my last tank, I got 21.6mpg - but this is what the display read, not my calculations, but still...I'd think you would get higher numbers with a n/a

I am surprised by how poor gas mileage you are getting with a N/A. In my turbo 4, I get between 21 and 24. If I baby it, I can get up to 24 - and I NEVER drive in 'intelligent' mode. When I have some fun, like my last tank, I got 21.6mpg - but this is what the display read, not my calculations, but still...I'd think you would get higher numbers with a n/a

Where/when do you drive? Sitting in I-95 traffic I got 18-19MPG, and that was almost entirely in I mode. Since I've been driving it more aound town (and usually more spirited), and when my wife takes it out we get 15-16MPG. Recently on a trip, keeping it 65-70, I managed 27-28, but that was entirely on cruise control!

I've noticed that if I park at night and the ave. is say 23.1 and I come out in the morining to start the car; sometimes it will have dropped by as much as .3-.6mpg just by starting the car. It's like someone came in during the night and and stole my MGP's!

I've noticed that, too...I give my wife a hard time about it: "See...your foot's so heavy you just lost milage!"

I've noticed that if I park at night and the ave. is say 23.1 and I come out in the morining to start the car; sometimes it will have dropped by as much as .3-.6mpg just by starting the car. It's like someone came in during the night and and stole my MGP's!

That can happen if you've recently refilled the gas tank. Cold starts do burn a lot of gas because the engine needs a rich air/fuel mixture when its cold and the AFR sensor isn't up to temperature.

Recently I've compiled my real mpg (full tank trip miles/gallons)..........

 

Unless you have checked your odometer for accuracy, you cannot assume your "real mpg" is more accurate than the mpg displayed on the dash.

;1665154']That can happen if you've recently refilled the gas tank. Cold starts do burn a lot of gas because the engine needs a rich air/fuel mixture when its cold and the AFR sensor isn't up to temperature.

Ya, I loose some while idling, but mine will also actually loose the .6MPG between the time I shut it off and turn it back on; it starts out .6MPH short and drops from there as the car idles. I'm sure it's just because it re-calibrates when the car is restarted, but it still feels like I've been robbed.

And yes that meter is real touchy on a full tank. I've pulled into my driveway a few miles after a refill and had it read 32+ ave. MPG and then start it in the morning and it will drop all the way back to 23MPG by the time I pull out of the driveway on those sub zero mornings . My average MPG right now is at a record low of 17.7 for this tank!

After a long highway drive my Computer is usually within 0.5 mpg of the actual mpg. Last trip it was dead accurate. Now if your tank is mainly all city driving then expect the computer to be 1 or 2 mpg optimistic.

Unless you have checked your odometer for accuracy, you cannot assume your "real mpg" is more accurate than the mpg displayed on the dash.

Well I would hope that Subaru can at least make an accurate Odometer since that is one of the most important things a car needs... but I think it is since the distance in my old car (Elantra) from my home to my college came out to be exactly the same as in this Subaru. The car is stock so no aftermarket wheels are screwing with what Subaru developed.

 

The reason my mpg is really low recently is weather fuel + cold weather + mostly city driving/traffic. Usually driving to college is mostly highway and clear. Right now I have it on 29.7mpg though that will probably go down by the time I get back onto the highway and then ill have to baby the car back up to 30mpg. Though ofcourse I'm actually only getting 28mpg. Sadly one mpg above the epa standards.

 

As others have said I usually notice Mpg going down 0.3-0.6 especially during a cold start where I try to wait for the Rpms to idle down to 1k. Even when its garaged at home.

 

I feel disappointed in how low this car's mileage is even after babying it. In my last car (05 Fwd Elantra 138hp automatic) it was rated EPA rated 29mpg though if I used the same slow pace of driving I would see above 40mpg on several occasions! I guess its the fact that I have a heavier car now, AWD, and bigger engine with more horsepower. Ofcourse its all a compromise of fuel efficiency for performance and safety though I feel that the balance is not equal. The legacy has a Sohc boxer (68hp/L) compared to the Hyundai Elantra's 2.0L inline Dohc with CVVT (69hp/L). I hope Subaru replaces its whole line with a modernized N/A I4 and uses its 5speed auto as the base transmission or I'm going to have to look outside of Subaru for my next car especially with rising gas prices. AWD is great for snow/rain but most days are sunny. At this rate I'll probably use the Legacy for bad weather and another make's car for clear weather driving since Subaru doesn't offer anything better currently along the lines of fuel efficiency than the Impreza (which is hideous and has the same powertrain that I have now).

 

Here's an updated list with 2 more entries.

Updated Jan 26, 2008

Date___ Station_ Price__ Gallons Total Miles__ Trip CMPG AMPG Diff

01/20/08 Forbes $3.139 11.765 $36.93 18039 245.1 20.8 21.6 0.8

12/31/07 Forbes $3.139 13.908 $43.66 17794 309.2 22.2 23.4 1.2

12/02/07 Forbes $3.159 13.638 $43.08 17485 346.5 25.4 26.7 1.3

11/11/07 Forbes $3.099 13.178 $40.84 17139 305.3 23.2 24.0 0.8

10/28/07 Forbes $2.839 12.462 $35.38 16833 317.9 25.5 26.7 1.2

10/14/07 Forbes $2.739 13.865 $37.98 16515 345.9 24.9 25.8 0.9

09/28/07 711__ $2.859 13.229 $37.85 16169 334.1 25.3 26.1 0.8

08/14/07 Forbes $2.899 13.065 $37.88 15838 ????? ???? ???? ???

08/08/07 Mobil_ $2.878 13.158 $37.88 ????? 242.5 ???? 27.0 ???

Uggghhhh, stop your whining please. There are a lot of factors that effect your gas mileage, most have to do with winter formula gas. I see a good 3-5mpg drop in the winter time, even with warmer temps but winter formula gas. I can see an instant increase in the spring when the summer blend hits the tanks.

 

FYI, my 26 mile daily round trip consists of suburban driving (nothing over 50mph) and I average 26mpg (summer) and 22mpg (winter) with a 2.5i 4EAT. In any case it beats out my previous 98 SVT Contour, which had an 8 year average mpg of 21.7mpg with a FWD, 195hp, 2.5L, V6, 5sp Manual, that took premium fuel vs regular for the Subie.

 

If your main goal was MPG, you should have done more thorough investigation before your purchase of a Subaru.

I guess its the fact that I have a heavier car now, AWD, and bigger engine with more horsepower.

 

I hope Subaru replaces its whole line with a modernized N/A I4 and uses its 5speed auto as the base transmission or I'm going to have to look outside of Subaru for my next car especially with rising gas prices. AWD is great for snow/rain but most days are sunny. At this rate I'll probably use the Legacy for bad weather and another make's car for clear weather driving since Subaru doesn't offer anything better currently along the lines of fuel efficiency than the Impreza (which is hideous and has the same powertrain that I have now).

 

 

OMG. (If I were a Mod, you would be banned so you could spend some time thinking about what you just said.)

 

The Leggy was never intended to be an econobox!! And yes weight and AWD are very hard on MPG. If you wanted a Hyundai why didn't you buy another one?

Sorry for the rants but I tell it how it is. I like the look of the Legacy (exterior/interior) so I guess that was a big reason I bought it and will be keeping it. I know its not an econobox but I expected it to have better fuel economy especially the relatively slow way I've been driving/babying it. What I really don't like is the powertrain (2.5i and 4eat), I'm sure I'd be happier with at least the GT and 5eat since the extra power would warrant poor mpgs. Or probably I'd need to see how well the 5speed N/A does though I don't know how to drive manual and wouldn't "learn" how to on a new car.

 

Sure Subaru makes great turboed cars but I believe they've left behind their "roots" in the 90's where they had base models that started in the econobox realm. You can't deny that the first Legacy was seen as another Econobox and now its evolved and upgraded into a midsized/luxury vehicle. As we can all see now, Subaru is going to do poorly if gas prices continue to increase and other manufacturers have smaller compacts to rely on to make up for loses in their SUV/big car markets.

You still seem to be missing the point here. Subaru is (at least in the US) a niche manufacturer appealing to people who want/need an AWD family sedan. They don't sell as many vehicles as Toyota, Nissan, Hyundai etc because the highly advanced AWD system on ALL their cars makes them significantly more expensive, heavier and less fuel efficient. Technically the boxer engine is as advanced as any I4 except for those with direct injection. If you compare the Leggy with its real competition, AWD sedans, not FWD sedans, it is as light and fuel efficient as any you can buy. The fact that it is better looking than most is a bonus. If you didn't want/need AWD the Leggy never should have been your choice. If you did want/need AWD then be happy with a great car and accept that it will never get the mileage of an I4 FWD 6EAT Toyota, Nissan, Hyundai. OK?

you *could * (if you were THAT concerned about gas mileage) convert it to either FWD or RWD and chuck the excess, unneeded drive train components to save weight and reduce friction.

 

this isnt a good comparison (apples to potatoes) but when i had a loaner 87octane (4eat)? 07 outback while my 93octane 5mt LGT STG2 was at the dealer's, i did notice a -2/3 mpg difference (19-21 [OB] vs 21-23 [GT] by both cars' computer) under the same driving habits during the two days i had it. meaning, i got better gas mileage with my LGT under very similar driving circumstances. so yea, subaru really needs to do away with the 4EAT, its madd archaic.

:eek:You guys are just as religious about your make/model as any domestic red neck gearhead jeez... can't I comment on a company that I've already paid about just as much as everyone else to purchase their vehicle? I don't like a car because of its name or brand, I think every make has a certain great model while others fail, in this case Subaru has moved up to a niche in the middle of the pack and does not have a model or powertrain offered in the US to compete in a time of rising gas prices meaning they better get their act together or they will continue to see declining sales even during the winter time when harsh weather is a key selling point for AWD.

 

What I'm trying to say is that Subaru seems to put more emphasis obviously on their higher end motors while their low end powertrains are many years old... from the specs I see in the "face lifted" 2007 Legacy nothing seems to have changed performance or mpg wise, all they did was change it cosmetically (for the worse in my opinion) and that is why I bought an 2006 rather than a 2007. I like the exterior/interior of the car and ofcourse I do know that AWD's have more powertrain loss relative to FWD/RWD (duh). I want AWD because I've had many bad encounters with hydroplaning even when trying to drive carefully in my other cars. I'm just disappointed in after driving very conservatively on the highway I see relatively low mpg on a relatively new car.

 

Let me sum this up: What I want to see in the future of Subaru's base models is a modernized 2.0-2.2 DOHC vvt (or whatever Subaru variation is called AVCS I guess) with 5eat standard. I'd be willing to pay for that. Sacrifice some performance for better fuel economy while still retaining the safety of AWD the N/A is all that fast, never meant to be and if I was looking for all out performance I'd obviously of gone with the GT. I wish it could all be inside the shell of my 2006 which I feel is in the generation that looked best.

 

They offer in Europe a 2.0R (DOHC 148hp/145tq) as opposed to the 2.5 (SOHC 170hp/170tq) we only get here as our "base", the 2.0 gets 74hp/L 73tq/L opposed to the 2.5's 68hp/L 68tq/L. Alongside a 5eat (with proper ratios) Subaru could TODAY offer a powertrain that can probably see an EPA highway of 30mpg (meaning that if you're extra conserve in driving there is some lee way to see even more on top of that say upwards to 35mpg+consistently). Compound that with implementing VVT or a Direction Injection down the line and the increased mpg should be more. Ofcourse there is extra costs involved but the net cost reduction in using a smaller engine combined with the increased cost in transmission may largely cancel out in a sense. Also mass producing only 5eat and getting away from the ancient 4 speeds will reduce costs overall if adapted throughout the model line.

 

Sorry this has gone way off topic from my original post, I'm just trying to reasonably defend my opinion rather than saying what you guys do like (ZOMG HE TALKED BAD ABOUT A CAR HE OWNS ZOMG BANNNN... childish... please leave that in other threads, not mine ). I'm trying to be serious about the whole fuel economy issue that our country is going through and things will change due to the new CAFE standards that all of Subaru's current model line up fails to adhere to.