OBC MPG accuracy discussion

[stephenkirsh]

So ever since I bought the car, I've been tracking the accuracy of the OBC vs actual MPG calculations. Overall, it seems the OBC is overly optimistic by 0.66mpg. I've been pretty consistently tracking this over the last 9 months between two Chevron stations. It seems they're about equal, and the fact that they have the "Certified by CA State Weights & Measures as of _ _/_ _ _ _ stickers on them, I'm assuming they're pretty accurate.

Here's my spreadsheet. My fuel usage picked up in November 2014, with my new job (commute moved from 2 miles to 31 miles one way). You can see my first trip to the pump at a difference of 1.5mpg when I had at least 4 different intake leaks!

My "Actual" is calculated by dividing the OBC miles by the number of gallons on my receipt. I never "top up" at the pump.

18831

So, what are you guys getting?

[theothersawyer]

ive noticed roughly the same, but I average much better MPG! I'm currently getting 29.5mpg this tank (200 miles on the tank so far) I average ~26mpg, but I have a 52 mile one way commute to school.

When I filled up last I reset my OBC and then drove about 60 miles home. The OBC said as high as 34.5mpg! That's the highest I've seen!

[az3579]

On my record run of 35.6 mpg over 100 miles, the OBC showed 36.0 exactly. That's a .4 difference.
Other times I've had about 33mpg indicated with low 32's as actual. It's usually between .3 and .8 difference for me.

[NorCalZman]

Ok glad it's not just me. I haven't been recording the OBC mileage like I have my actual, but I have noticed it. It is consistently ~1 mpg more optimistic than my actual. I think that's a pretty large difference over a tank of gas. Wonder why it reads a higher mpg.

[BMWCurves]

In my 03 330Ci the OBC MPG was usually about 0.5 MPG optimistic compared to the actual MPG. The highest I ever got was 30.2 MPG actual, and the OBC was under by about 0.6 MPG.

[stephenkirsh]

Wonder why it reads a higher mpg.

I was hoping this thread would help us determine that :)

It's a bit confusing considering how many computers and sensors these things have. But you can see how bad the difference was when I had 4 intake leaks: 1.5mpg! Maybe there's no way for the computer to directly track fuel usage and it just makes a guess.

[TigerTater]

Yours is similar mpg to mine. Autos don't get good mpg ESP when you drive the zhp like its supposed to be driven!

[theothersawyer]

Just filled up today and the OBC said 28.7mpg and then the calculated mpg was 27.2 that is a significant difference of 1.5mpg!

[stephenkirsh]

That's pretty big. Any possibility of intake leak(s)?

[theothersawyer]

It is the largest difference I have seen since owning the car. Nothing has changed since I bought it, but I will definitely check the intake boot. I have yet to check the condition of that since they are known to go bad.

[stephenkirsh]

Could have been a fluke, too. See if the next tank is off by as much. If so, probably time to do some investigating.

[theothersawyer]

Will do! I was really hoping for great mileage this tank, and have been really trying hard to get as close to 30mpg as possible. I was okay with the 28.7 that the OBC stated, but then was disappointed when I calculated it out and only saw 27.2!

[quikryptonite]

I'm usually .25-1.00 gallons off, typically not in my favor. If I am just commuting on the freeway the OBC MPG and actual MPG are usually pretty close within .25 MPG. When I stomp on it on the on ramps and curvy interchanges my OBC MPG usually is off a lot more than just normal driving. ;)