It seems my car is overstating the MPG, at least on my last two tanks, which are the only ones I've checked. I'm getting low MPG in general (as discussed in another thread), and wanted to double check that the car is accurately calculating the MPG. On each of my last two tanks, it has overstated my MPG by more than 2 MPG. For example, on my most recent tank it says I achieved 28.6 MPG for the tank. But I went 301.6 miles on 11.378 gallons, and unless my basic math is way off, that would be 26.5 MPG -- more than 2 MPG less than the car says. The same thing happened on the tank before.

Has anyone else had this issue? If you don't double-check your car's math, could you do so on your next tank and let me know if you're having the same thing happen? Thanks!!

 

Bad news and good news. First the bad news. Most people find that the TCH's mpg display is higher than what is calculated using the odometer and fuel pump reading (although I've found it to be more like 1 mpg).

Now the good news. I read a thread about a month ago where some people used their GPS to calibrate the odometer. They found that the odometer was reading low. After taking that into account it appeared that the TCH's displayed mpg is accurate.

I don't own a GPS so I have no way of verifying this.

 

Terry,

Check out this thread for additional discussion:

http://tinyurl.com/yymngy.

And I think Marc was referring to the following thread discussing odometer error as determined by GPS readings:

http://tinyurl.com/yxq2zh.

 

Thank you!

 

Huh. All my 19 tanks were manually calculated using the car's odometer and the actual amount of fuel pumped. I wonder if it's low? I should take a highway trip and compare the mile markers to the car's measurements, since that's by far the most accurate measurement, or at least that's reasonable to assume.

 

Face it folks - there will NEVER be a car computer that COMPLETELY NAILS the EXACT MPG you get for a tank. If it happens, meaning the fuel display computer exactly matches your miles-driven-divided-by-gallons-pumped calculation, it will be a complete accident.

To my knowledge, there is no vehicle device on earth which actually MEASURES the amount of gas burned to come up with it's calculations.

Being a mile or two within the gallons pumped calculation is about as close as we can get. Sorry folks, but that's the current reality.

 

The measurements I did show that the odometer is reading about 1.4% low. Another poster came up with 1.5% low. Meaning when the car REALLY went 100 miles, the odometer said I went 98.6 miles. This will cause your MPG to read 1.4% lower than it should. For me, that meant my calculations said I got 36 MPG, but the real number should have been 36.5 MPG.

That amount of error is small and can be easily overwhelmed by how you top off your gas tank, and the temperature of the dealers gas in their underground storage tanks, etc.

So, it isn't something to be concerned about. Unless you are a fanatic about getting the highest MPG in some sort of competition.

 

Go to this thread for mileage & speedo error:

https://www.greenhybrid.com/discuss/...ror.10105.html?

 

My GPS is 2 mph faster than my Speedo

 

Using the car’s calculated mpg reading is an estimation and does not tell you accurately about the long term mpg. Using just one or 2 fill ups and manually calculating the mpg can lead to errors due to fill ups. But keeping a records of fill ups in an excel spreadsheet can give a more accurate long term views and the errors tend to equalize out.

Here is one that I have created.

Remember

• I am in the medical field and not in accounting and have only limited knowledge of Excel. If someone can modify it to make it more efficient you may.
• All you need for the records are the small tear away print outs that you get when you fill at the pump using your credit card. It contains all the information except the odometer reading. It would be nice to record it each time but it does not matter since only the last odometer reading is needed for calculation of overall mpg.
• The last odometer reading divided by the gallons filled just before the current fill up gives an accurate mpg. In my spreadsheet the value of 26.98 mpg is inaccurate since it shows the 14 gallons filled on 10/14 but does not show the miles obtained from it. Either I should wait for the next odometer reading before the next fill up or remove the 14.00 gallons to show the last accurate mpg of 34.59 mpg.
• The cost per gallon is the total amount paid divided by the total gallons filled.
• You can insert rows above the last column in order to increase spaces to add data should 29 fill ups readings not be sufficient.
• I have 2 gps usb receivers and 2 software programs and both show a 4 % error in the speed. ie. 70 mph on odometer is truly only 67 mph. Both my Accord and Odyssey are spot on the gps readings. But I have yet to do a “gps trail” calculation if the odometer reading is also inaccurate. I will let you know after I do a decent sized trip to see its error margin.