Totally inaccurate mpg readings

[427435]

Over the last 10 tank fills and 2804 miles, I've been tracking the fuel used and the mpg readout at the time of the fill. I then multiplied the miles driven by the indicated mpg for each of the fills; added these numbers together; and then divided by the total miles driven.

That gave me an indicated (per the gauge) mpg of 20.97.

Dividing the miles actually driven by the gallons actually used gave me a mpg of 19.19---------------or an error of 9.3%!!!!!!!!!

I understand a little fluff in the computer program, but this is ridiculous.

My two 2003 vehicles with mpg readouts are within 1-2%.

Shame on Hyundai!!!


For what it's worth, I'm OK with the actual mpg----------I just don't like being lied to.

 

[KyKernel]

I noticed the same thing over several fillings. Although I have not kept records as carefully as you, a 10% error would not surprise me.

 

[videopho]

"My two 2003 vehicles with mpg readouts are within 1-2%"
Dunno what vehicles you were referring to but it seems impossible to be that accurate unless the car was running on lift.

 

[Mark_888]

Dividing the miles actually driven by the gallons actually used gave me a mpg of 19.19---------------or an error of 9.3%!!!!!!!!!

I understand a little fluff in the computer program, but this is ridiculous.

You can't take the car salesman out of these guys. One of the fundamental principles of selling cars and keeping the customer happy is making sure the customer is satisfied with his purchase. In terms of price, that means it is not important how good a price the customer actually gets, but how good of a price the customer "thinks" he got. Fudging the MPG calculator is just a logical extension of that idea.

 

[wlk12]

Can't speak for a Hyundai, but my Lexus IS250 is constantly high by about 10%. Wife's new Cadillac ATS appears to be right on.

 

[Ernie3647]

Mine has always been 2% to 3% from actual. Must say “That’s good for me”.

 

[427435]

"My two 2003 vehicles with mpg readouts are within 1-2%"
Dunno what vehicles you were referring to but it seems impossible to be that accurate unless the car was running on lift.

The vehicles are in my signature.

There is no reason with the computer controlled fuel system, that the read-out can't be quite accurate. It's just a matter of programming as the computer knows exactly how much fuel is being delivered.

 

[Alwaysforward]

I have a fuel management system on my boat that is accurate within 1 tenth of a gallon very time. Its made by Yamaha and you can even make minute adjustments if required to dial it in. I am quite sure these fuel management systems in cars can be adjusted some how.

 

[landtuna]

That gave me an indicated (per the gauge) mpg of 20.97.

Keep in mind that the gauge resets itself when gas is added to the tank. If you are using the gas gauge number you must read it before you fill up.

This isn't a problem if the gauge is set to "immediate mileage" but can give you a faulty number if set to "continuous mileage".

 

[landtuna]

There is no reason with the computer controlled fuel system, that the read-out can't be quite accurate. It's just a matter of programming as the computer knows exactly how much fuel is being delivered.

Measuring fuel delivery is quite easy and accurate. Measuring distance, the other component of the mileage calculation, is not. Things like tire wear, non-standard size and even driving on slippery surfaces will affect that measurement.

I'm guessing that it is the distance measuring component that is faulty and not the fuel delivery. Perhaps Hyundai should have used GPS as the measuring device and not the ODO.

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[brfatal]

Your tires alone can do this as they are not all the same circumference brand new off the factory floor nor from the time you install them to the time you need to replace them.

Fuel economy computations are always going to be estimations and should always be taken with a grain of salt because they are never accurate.

 

[thrawnis]

My 2010 4.6L seems to always be at least 10% off. It is generally off between 2-4 mpg every single time.

I'm happy to see that I'm not the only one seeing this problem but it's unfortunate that it is so off.

 

[GenesisCaptain]

Hyundai is choosing to register high, pure and simple. It is not an accident.

The engine computer knows exactly how much fuel the injectors dispense, and it knows exactly how many times it fired the injectors. With a quick multiplication, it knows exactly how much fuel overall was expended. The OBC can get the mileage from the odometer and divide that by fuel dispensed from the engine computer. The answer should be within a couple percent if we did the same math using fill-up and observed odometer mileage.

Mine is 15% high. Hyundai's choice.

 

[427435]

Keep in mind that the gauge resets itself when gas is added to the tank. If you are using the gas gauge number you must read it before you fill up.

This isn't a problem if the gauge is set to "immediate mileage" but can give you a faulty number if set to "continuous mileage".

That's what I did. In fact, the "old" mpg stays on the gauge, after you fill, until the car starts moving.

 

[427435]

Your tires alone can do this as they are not all the same circumference brand new off the factory floor nor from the time you install them to the time you need to replace them.

Fuel economy computations are always going to be estimations and should always be taken with a grain of salt because they are never accurate.

OE 19" tires with 8000 miles

 

[landtuna]

OE 19" tires with 8000 miles

Are they filled precisely to the factory recommended air pressure?

Do they change circumference as you drive by heating or cooling?

Several other things to factor in.

 

[Gen 4.6]

Doesn't mpg also depends on how we drive ...lead foot or not?

 

[veed]

mine (2011 4.6) has never been right. has read as high as 31+, and as low as 21, but is almost always 22-23 (manually measured per fillup by gals added and odo).

there was some kind of gas card thing back when by hyundai as a "so sorry" for crappy calcs, but it only applied to certain models. can't remember details - it's in here somewhere.

 

[427435]

Are they filled precisely to the factory recommended air pressure?

Do they change circumference as you drive by heating or cooling?

Several other things to factor in.

Radial tires change little in circumference due to air pressure.

 

[427435]

Doesn't mpg also depends on how we drive ...lead foot or not?

Yes it does-------------which is why I would like an accurate reading.