Over the weekend I did an all-highway trip in my new (to me) 2010 non-turbo R56, and got what I would call "terrible" average gas mileage. The computer claimed I got 40.0 mpg, but when I filled up and calculated based on the trip meter, I actually got 31.8 mpg. (I always reset the trip meter and average mpg at every fill-up)

My first question is this; does 31.8 mpg sound like a typical result for the non-turbo R56? I spent 95% of the trip in sixth gear with the cruise control on an indicated 75mph (I think my speedometer reads 2-3 mph high). The air filter is brand new, engine and gearbox oil are new, the tires are properly inflated, I don't have any aerodynamics-killing body mods, and I wasn't running the air conditioning. I ran the vent at the lowest fan speed on the windscreen the whole trip, I don't know if that runs the A/C enough to destroy the fuel economy.

My second question is in regards to the 40.0 mpg the computer claimed I got. How can it be off by so much? Depending on which way you do the math, it's reading 20-26% high. I have been experimenting with the CONSP CORR section of the hidden computer menu, and during this trip it was set to 0900. I think that's supposed to change the displayed mpg by 10% from the default setting of 1000, though I'm not certain which way it should change it. If we assume it made my displayed mpg 10% optimistic, then that only explains half or less of the discrepancy seen. As a result of this enormous error I have changed CONSP CORR to 1100, but in the 100 miles since then the computer is telling me my average mpg is over 42.

Lastly, has anyone seen their displayed and/or actual mpg be very good during the first half of a full tank, then much worse over the second half?

 

Over 21k miles my base 2010 overstated the mpg by an average of 5.5% with a range of 1.5% to 15.6% difference per fill up. Actual mpg was 35.7 over the total distance with a minor GPS correction of the odometer. It is not easy to "fill up" the tank to the exact same level from one tank to the next. Average a few consecutive calculations and look at that.

I have no idea if or how your tampering with the code may be affecting the computer calculation of mpg.

 

I drive an 2010 MCS and get considerable better calculated mileage under those conditions, so I'd say something is not right with your car or the calculation.

The dash display overstates mileage but, again, nowhere near as much as you experienced. I calculate mileage at every fill up and the dash reads 2-3 mpg high. My odometer appears to be very accurate when compared to GPS.

I've never noticed any obvious change in displayed mpg accuracy over the life of the tank. The range display, now that is another story.

 

My mpg estimations have always been hilariously off. Though my actual fuel gauge has never quite worked right either. When the car was new I went through three different dealerships, they tried everything and could not figure out why it wasn't working. It will show 3-4 bars when it's empty, or show it's empty when it's only a gallon or so below full. The day I got the car I actually drove it dry out of fuel because of that (had no idea at the time what to expect from one full tank).

 

Yeah, mine changes significantly too. At the start of a tank it's usually claiming I can go 580-590 miles. By the time I fill up near the end of the tank, it's down at least 100 miles.

I think I almost suffered the same fate for the same reason. I ran the tank down until only two fuel lights remained. When I filled up, I managed to put in 13.5 gallons even though my tank supposedly only holds 13.2. I must have been running on fumes.

 

The range display shows how many more miles you can drive before you run out of petrol. It should go down continuously as you drive until you add more fuel.

Stated fuel tank capacity typically does not include the filler pipe.

 

I am guessing about .3 gallons in the filler neck, so going off that, I got about 27.9 mpg. It was on a trip that was about 90% highway driving. A/c was on about half the time. Again this all a rough estimate.

 

I almost always get great mileage with my car unless I'm going 80+ for a whole tank. I can easily get over 500 miles on a tank and my best tank was 632 miles driving to and from Copper Harbor in Michigan's Upper Peninsula...where the speed limit is 55 and I just put the cruise on 60.

 

the epa number is 33 highway 2010 non turbo automatic. 32 is hardly something to be upset about.

 

The OP has a manual. 37 mpg highway.

 

The problem that I experienced in the first post in this thread got radically worse on my most recent tank of fuel. This time the driving was only a third highway, the rest on country roads in Vermont (where economy usually improves for me). The computer claimed I got 43.1 mpg, but I actually only got 30.1! Like before, that's surprisingly bad actual mileage, and a staggering 43% error in the computer estimate!

The error in the estimate by the computer appears largely or entirely out of my hands. No matter where I set "CONSP CORR" the estimate stinks. But the actual, measured gas mileage is something that I should be able to improve. I'm not getting any codes, so is there a chance a worn O2 sensor could be causing my fuel economy to stink, but at the same time it doesn't throw a code? If I replace them, should I replace both or just one of them?

I just noticed on ECS Tuning's website they have this comment on the O2 sensors they sell:

I think I might replace them.