First 10,000 miles with my 2012 Prius

Do you believe the odometer to be correct as to how many miles you've driven?
Do you believe the gas pump to be correct as to how many gallons of gas you bought?

If you believe those to be correct, you have to believe you can deduce a correct MPG over time.

If you do not believe those to be correct, you will not believe you will ever deduce a correct MPG over time.

It really does break down that simple.

 

I believe neither the pump nor the odometer to be exactly accurate, as too many variables can impact either measurement. In fact, I would be surprised if the calculated MPG matched the car's reported MPG, as different variables impact each. But, I also believe that either is "good enough."

 

Your whole response is implying that I said manually calculating your miles is the 100% accurate, non-flawed way to calculate MPG.

I never claimed manually calculating is completely 100% accurate. I am stating that it is more accurate than the Prius dash reading AND we should be skeptical of the dash reading and not assume it is more accurate.

"Look... the car is probably off as well". Exactly

 

How hard would it be for a skeptic to fill a container with a couple gallons of fuel on a normalized STP day and weigh it ?
Or check the odometer vs a GPS or a good map ?

I'll bet a lot of money the difference is a whole lot less than the 4.7% average difference between pump calculation and car reading.

 

Fair enough.

However, for myself I believe this method to be far more reliable than the MFD since both odometers and gas pumps are by law supposed to meet certain requirements. Yes, in the short term there will be variables that throw either number off, but in the long run those variables tend to "even out" over time.

 

Face it, while we calculate miles per gallon, really that number is not the one that really matters - it's miles per dollar. And that means that the pump is dead on, because that's how much the company is going to ding my credit card.
The only reason mpg makes sense to track is that the price varies so much and we strive to be able to compare day to day results in order to track variables like tires pressure, weather, and driving style.

 

That's what those little certification stickers on the pump are for - the state inspector did just that.

 

What is wrong with measuring the amount of gas at the pump (by the pump) and measuring distance with car odometer. Everybody has use of the gas pump and everybody (and the car MPG calculations) uses their car odometer for distance measurements. In your own words it is "The only way to get an accurate MPG... actual amount of gas ... measured distance." Over multiple fill-ups it doesn't matter how you filled the tank up, or even how full the tank was before or after the fill-up. As long as you have an accurate gallon amount measurement from the pump and the odometer reading you can calculate accurate MPG

I would claim that a bunch of people postulating that the car is high is based on the only way to get an accurate MPG, not anecdotal evidence, rumor or hearsay. Both the car and owners calculations use the odometer for distance, so that only leaves the amount of gas as a variable. Pumps are engineered to be accurate and they are certified by the state, so I would say that they give an accurate measurement. So that only leaves the study of how the vehicle's MPG calculation relates to the calculated (amount at pump/odometer distance) over time. I think that the "bunch" of people have answered that question.

I invite jabecker to register at fully.com and start your own study of car vs. calculated MPG. My experiment using fully.com (and spreadsheets) shows 2.5 MPG (%5.87) higher MPG reported by my Prius v with 4359 miles of good data. I try to add a graph later.

 

For the my last 4 tanks totaling about 2,400 miles, the display reads 5.52% high ... but who's splitting hairs!!!

 

The odometer is almost certainly not accurate. It's influenced by factors such as tire wear. Even being off by just a little will impact the calculated results. That's why I said that to be absolutely accurate the distance had to be measured by some other means.

Why do you assume that I don't track my gas mileage, simply because I don't use Fuelly? And, by the way, Fuelly only shows the calculated mileage, so comparison with the car's reported MPG isn't possible for anyone but the owner.

I've filled my car three times since I got it, and I have about 2000 miles on it. I threw out any calculations from the tank that came with the car, because I had no idea if it had been driven since filling (my car came from another dealer in another town and both trip meters were reset by the dealer before I drove away in it). For the other two fill-ups, the calculation was once high (58.7 MPG calculated vs 56.4 MPG displayed), and once low (47.3 MPG calculated vs 52.6 MPG displayed). That's a difference of 2.3 MPG and -5.3 MPG. [eta-may not be exact on the displayed as I'm going from memory. will double-check when I get home.] Both are probably within the margin of error. I'm guessing that over the years the totals will be pretty much a wash.

[edited to fix numbers that were slightly off]

 

Fwiw I think any discussion in this particular thread will be entirely fruitless. You are clearly not going to change your mind. And we sure as heck aren't going to change ours. Just my 2 cents though.

 

Read more: ​

First 10,000 miles with my 2012 Prius | Page 2 | PriusChat​

Sure, but the better question is HOW MUCH. E.g, take a tyre that spins 600 times a mile. The radius is about 17 inches. Then using 1/32nd of tread changes the odometer by 1/17*32 = 0.18 %​

Fuzzy posted 0.2% inaccuracy on the odometer for tyres with 20k miles wear. I read on wikipedia that good gps devices with odometers are accurate to within 1/100th to 1/1000th of a mile. For government work, figure you know a 10 mile ride to within +/- 2.5 ft.

Knock yourself out

 

I would agree with that assessment. See... we can find something to agree about.