Question wrt onboard MPG calculation

I think mine is 4% high, but the math is simpler if I subtract 2 MPG

 

You can use Fuelly to track your mpg. Just record your odometer reading at each fuel up and enter on the website. Fuelly will calculate everything, even graph it out, so you can see trending.

 

Fuelly is forgiving of occasional misses, it just starts calculating again. If she does the fill ups more than 20% of the time, it would be poor accuracy.

 

You can also update Fuelly with a very simple text message.

 

Its actually VERY accurate, but based on an inaccurate odometer. The speedo is about 3-5% fast (depending on actual speed), which is why the MPG displayed is also off by the same amount. It's pretty consistent, so assuming its 2MPG off is reasonable if you're in the 50MPG range. Again, how far its off will depend on the average speed for the sampling period, the higher the speed the further off it will be.

Or use the Android app.

 

I use Fuelly too, but it's only as accurate as the data it is given. I particular, the miles driven figure is what the car's odometer is reporting. If the car is slightly under-or-over-reporting the distance, then Fuelly's MPG calculation will be off by at least that same amount. Also, we don't know how accurate the "gallons dispensed" figure that the gas station's pump reports is, although I'm sure there is a governmental agency that controls this. Therefore, I would suspect the car's mileage figure more than the pump.

 

Gas station pumps are mainly regulated by states. Enforcement is slack at best, nonexistent in most cases. Independent and state-sponsored studies of gas stations have been done, revealing that the gas itself is sometimes not what's advertised, and pump accuracy is a moving target as well.

Here's a state survey done in AZ last year:

This is why its recommended that for accuracy you always fill up at the same pump.

 

Wow, very interesting. I guess I was just being naive expecting that oversight by a governmental agency would assure accuracy. In hindsight, when has it ever?

 

I think the state inspections DO protect you from systematic 'shorting' the customer. Errors around a bell curve of the right answer are more forgiven. In the example 12 were 'shorting' the customer, 27 were in the customers favor, not a case of fraud. (and 100 were right on)

 

I think the measure of "accuracy" is something like 0.5% plus or minus. Sometimes measured in cubic inches per 5 gallon test.

Lets not get into expansion and contraction with temperature.

 

What evidence do you have that the odometer is off, and how far off is yours?

The odometer and displayed speedometer errors are not the same.

Many of us have found (via ScanGauge and other engine monitors) that the ECUs internally know the speed fairly accurately, but the dashboard display number is bumped up higher. The odometer doesn't display a similar error. Both my Prii display(ed) speeds 1-2 mph high, but have(had) odometers displays reading 0.2-0.3% low.

My other past cars also had odometers reading much more accurately than their speedometers.

If your odometer is really reading high, then go find the class action product liability lawyers who hit up other car makers on similar claims. Both my Honda and first Subaru were covered by such 'warranty fraud' suits, despite mine not having the alleged errors. The lawyers got a big paycheck, the lead plaintiffs got some money, and the rest of us got a 2% mileage warranty extension, on what have been my most reliable cars ever.

The lawsuit fallout shows on my new Subaru: its odometer is reading 1.8% low.