I wondered why my gas mileage was erratic. Except for 2 fill-ups, I used the same gas station every time. This time, I had "20 miles to empty"... which I knew from experience translates into having more than 2 gallons left in the tank. But when I filled-up the car, the pump stopped at 19.951 gallons. NO WAY. The Altima only holds 20 gallons and the most I could possibly pump would have been 17.951 gallons if I followed my "2-gallon remaining "assumption.

If I used the gas station gallons to calculate mileage, I got less than 30 mpg (29.xx mpg). My on-board mileage computer said 33.4 mpg. If I used "my guestimate" of a true 17.951 gallons, I got 32.56 mpg.

I am not filling up at that gas station anymore.

 

I was traveling to Texas one time and found a gas station that was much cheaper than others nearby in AZ. I found out why after filling up, the pump said I pumped 17 gallons into a 15 gallon tank, with a mpg reading on the computer at 36, but hand calculation at about 30... definitely something fishy going on at that station. Granted I could possibly get an extra gallon into the tank, not two- no way was I close to zero at fill-up, and the mpg on my computer is generally close to my hand calculations.

 

Every pump and all odometers have a margin of error, about 3% to 5% plus or minus. This is why I trust the computer more than hand calculations. The government checks the pumps, but doesn't let the people know who is not in compliance. Consumer reports never trusts odometers and always uses their own. You can use an advanced GPS to try to figure your odometer error but GPS units have margins of error as well.

However.......the countdown in the NAH is always off. I have gone 40 miles beyond the countdown and still had over 3 gallons in the tank.

 

By law, at least in Florida where I live, both must be calibrated within one percent (1%). I think most other states have the same requirement.

If a gas pump is off by more than 1%, it fails the state inspection. The state inspector red-flags the pump, and it must not be used until it is repaired and retested. If a gas pump fails twice, the owner of the gas station pays a fine, and the pump must be completely removed and destroyed. Repeat or flagrant violators can go to jail.

Odometers must be accurate to within 1% when the car is new and the tire pressure is as specified by the manufacturer. Of course, odometers can be wrong after some tire wear occurs, or after new tires are installed. That's part of the reason the number of revolutions-per-mile is often provided in tire specifications.

 

And, every state has an enforcement department. If you suspect that a gas station is mis-measuring the fuel, just contact the enforcement department in that state. They will do an unscheduled inspection.

In Florida, contact information for the office of petroleum inspection is at:
http://www.doacs.state.fl.us/standar...o/contact.html

In California, it seems to be done at the county level. In Orange county, see:
http://www.ocagcomm.com/wm_petroleum_products.asp

 

^that's what i was going to suggest, call the cities weight and balances department

 

What is ideal and on the books rarely reflects reality. I wish pumps and odometers had just a 1% error, but there are way too many factors and variables. The government can check the equipment but can't test the variables. A 3% error is very conservative. Remember, I'm talking about the combination of pump and odometer error which needs to take an account of: tire condition, road condition, temperature, amount of additives, amount of fuel in the pump hose or fuel line, pump condition, pump age, amount of vandalism and a host of other little things.

If you find a flat straight road way with mile markers and stop at one, set your trip odometer and your GPS (and scangauage if you have one) and drive to the next marker, the readings will be close but slightly different.

 

Highway mile markers can be erratic individually, but tend to be accurate on average and over long distances.

Odometers are mostly consistent, any error being constant per mile. That is, if it reads 10' more or less per mile, then it will most likely do so for every mile. Small variations will occur on different road surfaces, tire inflation, etc.

GPS is extremely accurate, never off by even 100', typically within 15' no matter where you are, or how far you have gone. However, that small inaccuracy is erratic. The problem with GPS is that you are measuring straight line distances from start to end, with errors accumulated do to small changes in the road path which are not properly accounted for on the internal map causing GPS measured distances to be less than actual traveled. The GPS calculates your distance from nearby know positions on the map, and adjusts the map distances accordingly.

-- Alan

 

If the odometer is off, then that also factors in to what the computer is telling you.

I'm not sure why you would think the computer provides a more accurate number, it's not like it's using some highly calibrated equipment.

The last three cars I have had had MPG computers. On average they were +1 MPG from a hand calculation and I consider my MPG to be the average of the two calculations.


-Bruce

 

Hi Bruce,

I'm not saying the computer might not be off, but it should be constant, because it is based on one formula for determining MPG. Each time you fill the tank and reset the computer it applies the same formula again. I don't know everything the computer uses to estimate MPG but if it has an error it will be consistent.

Manual calculations always have two variables, the odometer and the gas pump readout. The odometer is reasonably consistent, but could be off by 1 or 2 percent depending on initial accuracy and the condition of your tires. The pump is different every-time you use a different pump and depends on the condition of the pump.

A 1 to 2% error on the odometer and a 1 to 2% error at the pump will result in a variable +-3% error. This explains why there is often a 1.something difference between the computer MPG and manual calculations. It also explains why there are not always consistent differences. If the pump was 100% accurate, the odometer was 100% accurate and the computer was 100% accurate they would always give the same results. If one of them had an error, but was otherwise accurate, the differences would always be the same. In practice, none of then are consistent because they all have a margin of error.

I believe, over time, the computer will be more consistent then the manual. Taking the average of the two is also a good idea.