I continually get a slightly different value when calculating my mpg myself than the gauge reading. Usually I get about 2 mpg better by my calculation. I'd like to believe that is true. I just take my total mileage between each fill-up and divide by how many gallons I put in - simple, right?

Not that I think it's that it's totally unexpected to get different results. I'm sure there is error in each.

But can someone explain which method is more accurate, why it is better, what the shortcoming are for both, etc. so I understand better.

 

I would say your calculations are more accurate since you know exactly how much gas you put into the car and how many miles you got on the tank that you just refueled. Miles divided by gallons=mpg

 

Yes it seems simple and pretty accurate.

But I do hear people sometimes say that there are some errors involved and just wonder what they are referring to specifically.

Also, what about the gauge? How does it calculate and why would it be less accurate, if that is the case.

 

The biggest error I have found is the difference in gas pumps. they each shut off as slightly different levels depending on the pump and other conditions... so the largest source of error in manual mpg calculations is the difference in the actual "full tank" level each time you fill up.

Some combat that by always filling up to the tippey-top of the filler tube every time so they're sure it gets filled to the same level each time. Trouble with that is that if you don't burn off that tubefull of gas right away you'll lose some gas from expansion/contraction of the fluid at varying temps - which might be a source of error itself.

I posted another thread on that very topic this morning. Maybe watch that one too for more info.

 

There are many ways that small inaccuracies can creep in to measuring MPG.

Gas expands when hot.
One gallon of cold gas will expand to become more than one gallon when it is hotter.
So if you filled up at 3 AM during a freezing night (even though the gas station's gas tanks are underground) you are getting gas that is more "concentrated", if you will.

Next the pumps are calibrated to some accuracy regulated by some governmental body.
I don't know what that accuracy spec is for but I'd bet the industry calibrates them at the side of the spec range that benefits them.

If the pump stops at, say, 10.00 gallons it did so because of some back pressure.
If you used another pump perhaps it would stop at 10.09 gallons because of its different calibration.
Nothing is exact.

An underinflated tire has a smaller diameter than an overinflated one.
This may be slight but it is real.
The only way the car "knows" how far it has traveled is by counting the revolutions of the tires.
I suspect the calulation assumes the tire pressure is 32 PSI.
A smaller (lower PSI) tire has to rotate more times to go a mile.

Air, like gas, expands when hotter.
The air in your tires heats up as you drive, so a lot of shorter trips will mean you are driving on relatively small tires - long trips on hot days mean you are driving on relatively larger tires.
It also with changes in ambient temp, and road temp.
This has to affect the diameter of the tire.
Again it may be small but it is real.

These things, and others, can throw off MPG calculations.

 

My first tank of gas was a fill to the top, because I filled it at home. DIC showed 45.8 MPG. Went 570 miles on 10.34 gals, low fuel light had just come on. Actual mileage with kid "topping" off the tank showed 54.9 MPG. Unfortunately we here in Oregon aren't smart enough to pump our own gas, as per our Oregon legislature. So I believe it will be difficult to accurately measure MPG. I stood by the kid when he filled the car, buuuutttt……. Current tank shows I’m getting 44.2MPG, but it’s raining hard everyday and traffic is slowed to a crawl during my 35miles to and then from work daily. I’m happy just knowing I’m getting great gas mileage, but would love to join the game of how to improve.



Great information on this forum, I’m learning every day.

 

Neither are we, in NJ. ::sigh::

 

What do you mean?
Here in California you can buy Self Serve (you pump it yourself) or full serve (gas station employee pumps it for you).
Full serve costs much more

Are you saying in some states ONLY full serve is available?

 

--

 

Kenny,



Here in Oregon we’re required to let, how do I say this nicely, people you wouldn’t normally let touch your car, flip open your fuel door, drag your gas cap across the paint, and slam the pump nozzle into the fuel inlet; and if your lucky they won’t spill fuel, scratch your car, or anything else while performing these difficult tasks.



I’m so happy… But it's a beautiful place to live....