Averaging 31mpg!

[KuroNekko]

I am averaging about 26 mpg with mixed driving in the city and highway. I have a 6 speed manual and use 6th gear every time I am on the highway above 60 mph.

ipaqxman,
I know you've had issues with your car. What have you tried to remedy it?
Have you tried resetting your ECU by disconnecting the battery for a while (I would recommend at least 30 mins.)?

Did you also check the spark plugs' condition? A faulty oxygen sensor will also cause poor engine performance and horrible fuel efficiency. It sounds too early to have these kind of issues in a new car like the Kizashi, but it's worth looking into.

[Juan]

the car average miles per gallon computer is a bit off compared to manually calculating the gas vs. miles driven - I have been noticing a small difference, the car display is about 2 miles off - when it tells me Im getting 31 mpg overall and I go fill up and do the math I find Im getting 29 mpg overall.

[KuroNekko]

But then you're assuming all the petrol pump bowsers are accurate too! I know here there have been tests carried out showing some aren't - even though there are laws governing them. If you fill up on a hot day (or with hot fuel) you also get less in your tank. There are so many variables.

I am aware that many pumps are off, but I actually rarely fill up at the same gas station. I get gas where I can, when I need and aim for reputable brands. This gives me a random sample of different pumps, but the car's calculator seems to be consistently off. I will keep observing over time.

My experience is similar to Juan's in that the car is usually a few mpg optimistic of actual miles divided by gallons. I also recall reading a review of the Kizashi by an auto critic where he also discussed the car's mpg calculation vs. the actual mpg.

[klement]

Same here ... the consumption is a bit of a lie...

It's always declaring lower consumption than what is real.

In my case the lowest difference is 0.13l/100km, the highest being 0.79l/100km, with average difference being 0.46l/100km and median 0.49l/100km. (statistics over 16 fuelings at random gas stations over 4 months).

[~tc~]

In my experience, speedos read 5% high pretty consistently across brands

[murcod]

But then you're assuming all the petrol pump bowsers are accurate too! I know here there have been tests carried out showing some aren't - even though there are laws governing them. If you fill up on a hot day (or with hot fuel) you also get less in your tank. There are so many variables.

I am aware that many pumps are off, but I actually rarely fill up at the same gas station. I get gas where I can, when I need and aim for reputable brands. This gives me a random sample of different pumps, but the car's calculator seems to be consistently off. I will keep observing over time.

My experience is similar to Juan's in that the car is usually a few mpg optimistic of actual miles divided by gallons. I also recall reading a review of the Kizashi by an auto critic where he also discussed the car's mpg calculation vs. the actual mpg.

Do a bit of research on how the amount of fuel you actually get varies with ambient and fuel temperature too. There are a lot of variables that will ultimately affect the calculated economy.

[KuroNekko]

There's been research showing that ambient and fuel temperatures have very little effect on fuel economy measures in cars. It's more of a myth that getting gas when it's colder is going to give you more. It may matter for a fuel station that is receiving 1000's of gallons, but to a car filling up less than 15 gallons, we are talking about an insignificant amount of fuel varying by thermal dynamics.
The myth comes from people believing that fuel stations receive their fuel in the early mornings so that the fuel is colder and denser and more can be delivered. The actual reason is just the same as bread in a bakery. You just stock up before your customers arrive to purchase it. I have read an article on this matter exposing this.
Although gas stations aim to get their supply in the mornings, it doesn't always happen. I am sure many of us have gotten gas a few times during the day when there was a tanker in the station.

I believe the car's mpg figures comes from calculations from the ECU based on speed, rpm, fuel to air ratio, throttle position, etc. Somehow, these figures just don't equate to mpg calculated by miles driven divided by gallons consumed. The fact that they are consistently different and consistently about 2~3 mpg off seems to be reliable data that the car's mpg figure is just high by a few mpg.

I can actually compare the car's mpg calculations with that of another device that connects to the OBDII port. It may to worth doing to see how these two measurements vary.

[murcod]

I'd imagine the fuel usage would be calculated from the known fuel pressure and each injectors opening pulse width. There shouldn't be anything needed to calculate usage?

[Ronzuki]

the car average miles per gallon computer is a bit off compared to manually calculating the gas vs. miles driven - I have been noticing a small difference, the car display is about 2 miles off - when it tells me Im getting 31 mpg overall and I go fill up and do the math I find Im getting 29 mpg overall.

In 2 yearsand 24,000 miles of driving the car this is the consistent result I've seen. With the exception of one time, it was dead on .

[Minority]

Speaking of GPS speeds, I noticed the other day my speedo reads high (normal for most cars) but I was surprised that it seemed to be out by 3-4km/h at 60km/h? Has anyone else checked their speedo against a GPS?

Mines out by about 7-8% (high)