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Old October 21st, 2019, 20:49   #1
jadney
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Join Date: Nov 2014
Location: Madison, WI
Default Gas Mileage: What my TDi says, vs reality

I've had 3 TDi wagons, a 2011 and 2 2015s. The 2 2015s were both purchased from a VW dealer after a Dieselgate buyback and modification.
For decades, I've kept a log of every fuel fillup, on every car I've owned since my '62 Beetle, so I could calculate gas mileage. I've always found this to be a good way to encourage good driving habits and to spot problems before they became obvious.
The diesel mileage that the 2011 calculated almost always matched the mileage I calculated, but the 2015s always display a mi/gal that is way optomistic. It's especially off at the high end. I've had a few tanks that the car tried to tell me that it was getting close to, or above, 60 mi/gal. When I run the numbers, the actual mileage is seldom above 50. On average, I'd say that the car's numbers are 5-10 mi/gal higher than actual.
I noticed this on our first 2015 and assumed it was just this car's sample variation, but that car got totaled by someone not paying attention to the road ahead of him. So we bought the second 2015. It puts out very similar numbers. It appears to me that VW is cooking the books to make it look like their modifications have actually improved the diesel mileage. It's not real.
What we're actually getting is maybe 42-45 on the highway. That's not bad, and I'm not complaining, but the 2011 proved that accurate calculations are possible and VW had the competence to do it. It appears that they have slipped up and failed to do as well with the 2015s.
We're on another long trip right now, and I just noticed another error. On the 2015s, I can look at 3 different mileage numbers: Since Start, Since Refuel, and Extended Period. If I fill up in the morning and then drive for a couple hours without stopping, the Since Start and Since Refuel should be exactly the same. They are not. I forget which is larger, but one was about 1.1 mi/gal larger than the other. That's WAY too much to be explained as a rounding error, or any other kind of excusable error.
There are clearly several problems with the calculation VW has programmed into these cars.
Has anyone else noticed this problem?
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