Instant MPG gauges are BS

"JCAlan" (jcalan) 08/05/2014 at 10:54 • Filed to: None

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Heres why: I haven't seen one yet that doesn't read "99" while you're stomping on the brakes. Yes, while coasting you're theoretically the most efficient. But braking is the most inefficient thing you can do while driving.

Think about it, you've already attained forward motion and now you're trying to stop it. That is the complete opposite of efficiency, yet your display is giving you atta boys for it. On a vehicle with regenerative braking, it's even worse because you're getting green bars or bonus points or whatever for braking. Even though you may be charging your batteries, you're still wasting the fuel that got you to speed whenever you're braking.

I think displays that train you to hyper-mile need to account for braking as a Bad Thing. Turn red, go to "0 MPG" or whatever. Has anyone ever experienced a car that does this?

K-Roll-PorscheTamer > JCAlan 08/05/2014 at 10:59

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My Astra's gauge/meter/counter only read 99+ when I was coasting; especially downhill. It read some pretty decently average numbers when braking.

Santiago of Escuderia Boricua > JCAlan 08/05/2014 at 10:59

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Technically it is still infinity because the car is moving forward with no power so no fuel is being consumed.

Also, mine reads 127.0. Engineers will get it.

dinobot666 > JCAlan 08/05/2014 at 10:59

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I drove a 2013 Subaru Legacy sedan a while ago that had some kind of eco gauge thingy. I glanced down at it once in a while, but it didn't really do anything to change my driving style.

daender > JCAlan 08/05/2014 at 11:00

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I just do it the old-fashion way since my NB Miata doesn't have such pointless apps. Take the miles driven, divide by the amount of gas you put it next time you refill. Bam, accurate mpg.

GTI MkVII > JCAlan 08/05/2014 at 11:02

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My Mk7 GTI shows "—-" when coasting, braking, or idling, and then calculates on the fly based on throttle input.

GTI MkVII > Santiago of Escuderia Boricua 08/05/2014 at 11:03

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Exactly. It's not an overall efficiency calculator - it's an instantaneous "fuel usage" gauge.

kanadanmajava1 > JCAlan 08/05/2014 at 11:03

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The long term fuel consumption values aren't necessarily very accurate either. My company did one field test with four Golf MkVII tdi models. We wrote down the average fuel consumption after each trip and drove 20,000 km with each. We measured also accurately the amount we put to the tanks. The true fuel consumption was ~10% higher then the value that the car gave.

One co-worker of mine has followed his car's fuel consumption and told that with his diesel Peugeot's real fuel consumption was only 1% higher than the car's own estimate.

Devlin Munion > JCAlan 08/05/2014 at 11:04

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I want to partially agree with you, yes braking is very inefficient and it would be better if gauges could account for this.

However saying that the gauge is wrong because it says 99+mpg while brking is also wrong, because from my understanding most instant mpg gauges only measure the fuel flow and maybe speed.

Which when you think about it, yes braking is bad, but all the computer sees is that you are using less fuel than you were before so it claims your efficiency is rising.

Should the gages be changed, proably but it doesnt mean they are broken or BS isnt exactly right either.

JCAlan > Santiago of Escuderia Boricua 08/05/2014 at 11:05

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Right, but if the goal of the gauge is to train you to drive efficiently, shouldn't it ding you somehow for rushing up to light and slamming on the brakes?

spanfucker retire bitch > JCAlan 08/05/2014 at 11:05

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If the gauge was about how much kinetic energy you're wasting by using your brakes, I would agree with you. But it's not.

Alfalfa > JCAlan 08/05/2014 at 11:05

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Well, you're still technically getting high mpg's the moment you're braking. You're eliminating your momentum, so your average mpg isn't as high as it would be if you had coasted to a stop. In short, while it doesn't always give you the most relevant information, it's still accurate.

Twingo Tamer - About to descend into project car hell. > JCAlan 08/05/2014 at 11:10

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They're pretty pointless although it's a fun game to get it as low as possible. I got mine down to 22mpg in hard acceleration even though my average is 53mpg.

davedave1111 > JCAlan 08/05/2014 at 11:14

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Yes. QED, that's not the reason the gauge is there. It's actually there to help you drive efficiently if you already know how, not to teach you if you don't.

Mind you, anyone too dumb to realise that after they brake, their fuel economy meter then drops right to the bottom as they accelerate back up to speed, will probably not learn anything from any system.

ColoradoTaco > JCAlan 08/05/2014 at 11:19

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It's just a fancy vacuum gauge.

ColoradoTaco > Twingo Tamer - About to descend into project car hell. 08/05/2014 at 11:20

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I've had mine read .9 while in full boost.

KusabiSensei - Captain of the Toronto Maple Leafs > JCAlan 08/05/2014 at 11:21

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Measuring economy is also BS. What you really want is the rate of consumption, as that takes DFCO into account.

If you have the option, change the units from MPG(US) to L/100km. You are now looking at a consumption figure, as opposed to an economy figure.

If you want to get really fancy, wouldn't you rather do it in J/100km? That being the amount of energy needed to move 100km at current consumption. If it went negative, that means you'd be regenerating energy on cars that have that capability.

Twingo Tamer - About to descend into project car hell. > ColoradoTaco 08/05/2014 at 11:25

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I don't think I could get mine that low in a 5000rpm clutch drop and rev to the top of each gear haha.

Saracen > JCAlan 08/05/2014 at 11:31

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It's an instant fuel usage gauge, not an efficiency gauge.

jariten1781 > JCAlan 08/05/2014 at 11:35

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What made you think it was a training aid? It either reads injector cycles or vacuum. It's an information aid.

There are some devices out there that use gps/accelerometers to determine your accel/decel rates and light up from green to red based on your driving efficiency. That seems to be what you're looking for, but it doesn't obviate the usefulness of the instant MPG gauge. In fact, you normally need the instant MPG gauge to train them to your car's profile.

LuczOr > JCAlan 08/05/2014 at 11:36

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The gauge is called "instantaneous fuel consumption," not "instantaneous energy efficiency." It simply tells you what your MPG would be if your current behavior continued for the rest of eternity. Since a lot of cars actually shut off the injectors on deceleration, you would use NO fuel if you continued to be in that condition forever. Unfortunately, the gauge only goes to 99 and not to infinity.

My point is, it's not an efficiency gauge.

llamaguy > JCAlan 08/05/2014 at 11:36

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MkIII Focus has this but it's not super useful.

LuczOr > JCAlan 08/05/2014 at 11:37

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I would say that the gauge trains you to use the accelerator efficiently. All other features are up to you.

davesaddiction @ opposite-lock.com > kanadanmajava1 08/05/2014 at 11:40

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BMW is also optimistic, but apparently there's a way to correct it in my car (not worth my time - I just assume the number it gives is always a bit higher than actual, and figure approx. usage in my head when I fuel up).

TwinCharged - Is Now UK Opponaut > JCAlan 08/05/2014 at 11:53

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The old BMW instant MPG gauge was fantastic. It was an actual analogue needle. A chef that my parents knew once took me to his home to give me an early Christmas gift. With him being a spirited driver, I always enjoyed watching that needle flip back and forth like a reed in a hurricane every time he floored and braked.

JCAlan > jariten1781 08/05/2014 at 11:58

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Then how would you describe the purpose of the instant MPG gauge?

jariten1781 > JCAlan 08/05/2014 at 12:12

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It's not the most useful thing out there and the vacuum driven ones (looking at you BMW) verge on useless.

Some situational uses for the injector cycle version: To tell you how much fuel is being consumed at any given time. The vacuum ones are pretty useless, but the injector cycle ones are decent. You can adjust speed to maximize steady state instant MPG while cruising, select the correct gear while hauling, etc.

Even when its reading no cycles (99+ in your example) it has some use. You can tell when the ECU has finished the cold/warm start cycle when you get it to peg while braking.

Remember, it's a free gauge on OBD II cars for the manufacturer. All of the data it uses is part of the gov mandated system. Since the car is going to be reading that data anyway, it's nice to be able to view it when it could be useful. You just have to understand what it's actually reading.

n54 & s38 > TwinCharged - Is Now UK Opponaut 08/05/2014 at 12:23

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I much prefer having an oil temperature gauge in its place...

MontegoMan562 is a Capri RS Owner > JCAlan 08/05/2014 at 12:39

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I like mine, I would have to hold the reset button to get it to re-calculate a 99 mpg, only time I've seen it was when I had cruise control locked going through some mountains, I went on a very, very ,very long descent on the far side and hit the re-calculate just for the fun of it, but it locked in at 99 up and down the next mountain, apparently I was efficient.

I have an 07 Mercury Milan V6 that it's in and I really have to put in effort to get it to act that way. It typically hovers around my combined city/hwy mileage of 23.