Car's computer agree with fuelly?

[Skydaver]

Originally Posted by Draigflag

Guys and gals, this article explains everything you need to know, no more speculation required!

Why Your Trip Computer Isn’t Giving Accurate MPG Readings (And How to Fix It) – Feature – Car and Driver | Car and Driver Blog

I'm off to read the article, but for now, I'll relate that my 02 Maxima reported 3-4 mpg better than actual, for the entire 145K miles I owned it.

If I was really working on economy (65mph on flat highway, and keeping it at 65), I'd get a reported 35mpg, and an actual 30.5 or so.

I will be interested in seeing how my new car does.

[Draigflag]

That sounds pretty normal, a consitant indicated reading from the cars ecu. I'm a bit confused as to why my last tank was VERY accurate, but could have been a small discrepancy with the fuel pump as my filling methods are very consistent.

[SteveMak]

My car's computer tells fiction when reporting fuel economy numbers. The indicated values are always wildly optimistic. I created a detailed fuel consumption spreadsheet years ago, and I started populating it along with Fuelly when I got my current car. Fuelly matches my spreadsheet's values, so I'm confident it's right.

Note that when you view a specific vehicle's Dashboard, it shows the fuel economy over the last 10 fuel-ups. When you use "Research Vehicles", it shows fuel economy since Day-1 for each contributing vehicle, same as in your badges.

[DesertRat2011]

The used 2013 Ford Edge I just bought is the first vehicle where the dash read out was the same as Fuelly.

[benlovesgoddess]

I wish I had created a dummy record for each car - to plot the fake results of the onboard computer.
Running alongside the real car, it would show the difference between true MPG and the one the manufacturer wishes you to believe the car does.
It should show the implied saving this trick gives, as I'd need to enter fewer litres to save the (almost always) higher MPG.
As the Prius has an onboard lifetime figure, I have a rough idea - let me check!

[Skydaver]

I found out that my Outback has a dealer adjustable setting that I'm going to have changes to be more pessimistic, that is, the readout is going to show lower fuel economy than it is now. I'm going to start by having them change it to 10% worse fuel economy. I think that will get it very close to actual values.

On this, and on the Maxima I sold, I also have an Excel spreadsheet that calculates my real fuel economy, and where I record the ECU reported fuel economy. I'll have to see if I can make a plot of that.

[benlovesgoddess]

For the Toyota works out around £40 over 7,000 miles. My old Hyundai was far more inaccurate - often nearly 20 MPG out, and usually around 10. For an accurate car, maybe a £1,000 swindle over its life, for an outright lying gas guzzler over £3,000 misrepresentation of how economical it is?
As just a tiny percentage of drivers seem to work out true MPG, the cars "over confidence" combined with the optimistic advertised urban/highway figures must create a reality where most people think their cars are far more economical than they are.
I bet they sleep better than I do, ha ha ha!

[Draigflag]

My last tank read 66.5 MPG, real was 62.9 MPG, I make that about 5% optimistic, or to put it the other way 95% accurate. Unless you use the same pump every time, and fill the fuel up the exact same that is until you can see fuel sitting in the filler, then your results are going to vary every time. I'm am very consistent with my filling methods, almost ocd.

[luckypants]

As stated elsewhere my Seat Leon's computer was about 8% optimistic, so I had the calibration adjusted to be more accurate. I was a bit enthusiastic so the computer is now about 1% pessimistic. I'm happy enough for that, it encourage usa to try to get better mpg figures.

As a side note, this correction can be done on all VAG group cars.

[Draigflag]

I wonder if it was part of the emissions scandal they purposely made thier computers read higher MPG'S? I wonder if that can be done for other cars too?