Yeah, 5-8 or so years is all I usually get too, although I am over 10 years on an Interstate motorcycle battery too. I don't know what makes the Motorcraft so special. The truck must have the perfect charging voltage, the battery must have the perfect design and high quality materials, and I must have the perfect use pattern.
Old EPA 23/33/27
New EPA 21/30/24
actually it can...my wife's camaro had some sensor issues during a low battery situation. it was confirmed by a data stream read out before and after a new battery install. apparently the computer was not getting enough "juice" to read, communicate, and diagnose accurately.
I had a similar problem on my 81 Buick years ago. There was an internal short to ground in one of the sensors. I figured it out because all the trouble codes thrown at me were sensors coming off the 5 volt reference on the CPU. I had to reset the computer and drive the car with one sensor unplugged at a time till I got the right one, and then I only had one code. Took a couple of days to figure out that one.
Perhaps a worn-out battery can lose efficiency, wasting more energy than a new one does during charge/discharge. If so, the alternator would have to do extra work, which of course is more drag on the engine.
However, 10% sounds quite severe. Do you have a fuel log with well-controlled accurate fill habits and a consistent route to rule out as many other variables as possible?
Hold on guys, I've actually had a similar situation to this. I had a battery that was dying if the car wasn't driven for a couple of days (89 civic wagon). The battery was just weak and crap. I swapped in a new battery and the lights got brighter and my mpg probably went up by at least 2 or 3, and this was mid winter. So climate can't be blamed.
After that I later pulled a sub box out of the back, wiring, amp, and more... All around lost about 150-200lbs of weight and gained another 2-3mpg, and even less strain on the alternator --- returning the car to factory condition. Even though I was at EPA with weight and bad battery... My city/hw mpg passed the EPA average(?) by 4-6mpg after weight and alternator stress lifted.
My mileage was degrading, when i popped the hood i found that the battery terminals where covered in green crap. Cleaned them up, oil and filter change and am running smooth as before. I will see after i get through this next tank. Battery is still good.
I charged and equalized the battery in the Volvo I'm driving and I found a 10% increase in my fuel economy. I think there really is something to having a fully charged battery. Charging the battery can help but if the battery cells are unequal as was the case with me, then equalizing them should help. I plan to get a battery desulphater to see if I can improve the battery's cranking amps.
^Interesting, I know my battery is somewhat low on fluid currently, it looks to have sweated some recently(possibly from the alternator having to run at full-recharge from jumping it off a while back now, 3-4 months). I never re-checked the fluid level under the caps, but did put it on a slow charge overnight about a month after the initial 'headlights left on for 2-3 hours bit'.
FWIW, this Civic calls for a 51R, and it currently has a 51 installed. Of course, the battery cables are connected to the correct post, but still makes me wonder.