gas milage

[Alan L.]

My GMC Sierra truck shows an average over the first 55K miles of about 18.8 MPG. Every time I have checked it manually its been 18 to 18.5.

 

[Moon]

I find the same small discrepancy to be understandable; but a 4 mile per gallon discrepancy seems way out of the bell curve ratio. I've no explanation, and from the replies no one else has seen such a huge difference in numbers calculated.
If I find an answer/explanation I'll be sure to post it.

Thanks all,
Moon of Ohio

 

[Moon]

Okay, I think I've figured it out ....but I'd like your alls opinion. I have a habit of leaving the car running if I'm in it. I hate to turn it off for short stops. I regularly have lunch in the car with it running. Going over reports before calling on a business, I keep the car running. So, I'd guess that is where the discrepancy comes in.
Sound feasable?

 

[MikePA]

<font color="blue"> If I find an answer/explanation I'll be sure to post it. </font>
Take a look at what can affect mpg

1. Brand of fuel
2. Driving conditions encountered between fill ups, both roads traveled and weather conditions.
3. Tank filling procedure.

Since the only 2 numbers used to determine mpg are miles, which is taken directly from the odometer, the place where 'errors' can enter the equation is in the number of gallons used. Does the pump shut off at the same level of tank fullness? Do you try and nurse in more gallons after tank shut off? If so, do you spend the same amount of time nursing fuel in?

For example, when I fill my K1500, I can get several more gallons in after auto shut off. So, if I calculated mpg on a tank where I quit filling at auto pump shut off and then figured mpg on the following tank when I nursed fuel in, I could easily see a 4 mpg difference.

 

[Av8r3400]

If you look at the trip computer during the idling time, on the "instant mpg" page it will show zero mpg. This 'should' account for the idling in the average MPG computation the computer makes.

These computers are notoriously inaccurate. 4 mpg would be within the b-r-o-a-d range of variability.

 

[Stonefox]

Moon, I know the trip computer in my car is wrong. I can have the cruise control set at 80mph and and zero the average speed while cruising at 80 and it will come back with an average of 40mph. I'm not sure if the speed is determined separately from the speedometer or if the programming code is wrong. I've done this several times and it is always off by a factor of two. Also the calculation for MPG is off. Also the computer tells me I'm getting 16mpg but when I calculate the MPG it comes out to 24-25MPG. The elapsed time is correct so i don't think it is a clock timing problem. Did someone leave the 2 out of 2 pi r ?
Stonefox
/forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif

 

[Moon]

Yep, mine is still off. But it's changing. I've gone from an all time high of 29.2, now down to 27.4. When I first got the car (brand new) it showed 24.? ...and stayed in the 24/25mpg range. Around 10k miles I switched to synthetic. Shortly after that I was reading 26mpg and it slowly rose up to 29.2. Driving conditions and such have not changed. I do 3/4 highway driving and that has remained constant. I haven't manually calculated it for awhile ... I'll do it next tankfull and see if anything has changed. Once the overfill shut-off clicks off the gas I quit right there. I only noticed a 1/2 gallon or less difference the time or two I did try to put more in after it clicked off. So, all tankfulls are not exactly equal, but I think their within 1/2 gallon of each other.

 

[fractal]

I recently bought a 2004 ford taurus. I was driving 600 miles a week, almost all freeway. It wavered between 19 and 20 mpg, low side when I was in town, high side on freeway. I even tried the hold the reset button for 5 seconds till it cleared trick. It still read around 20 mpg on freeway.

I saved all my fuel recipts for a month.

2597 miles / 96.xx gallons = 24.2 mpg.

The onboard computer was wrong.

I would fill up at a truck stop, hop on the freeway, set the cruise control and drive 3 hrs on flat road then buy more fuel just to compare the results.

Computer said 20.0 mpg. Gallons purchased / miles driven was 26.4.

The car computer is good for giving you an estimate of your milage and miles to go but I sure wouldn't bet on it being within 50% of being right.