I purchased a slightly-used 2014 Nissan Pathfinder in January 2015 and have been using it to commute to work (almost exclusively, with the exception of the occasional trip). It currently has 41,000 miles on it and had it's 30,000 mile maintenance on-time and I have been getting oil changes every 3,500 - 4,500 miles.
The majority of my commute (mileage-wise) is highway and I drive 112 miles a day. With the exception of the occasional bad-day traffic-wise, generally speaking the commute is relatively the same (56 miles and approximately 75 - 105 minutes each way, with 90 minutes being the norm).
Because of the mileage and time spent driving, I watch my RPM's and the digital avg MPG gauge pretty carefully and over the last 6 months, the avg MPG has only increased and pretty substantially. When I first purchased the car, it was slightly used and was at approximately 18.5 mpg and I could get 3 round trips to/from work before having to fill up and now it's at 22.5 mpg and up until about a month ago, I was able to get 4 round trips. This jives almost perfectly with the vehicle specs (19.5 gallon tank, fuel economy of 20mpg street - 26mpg highway) and my commute (4 roundtrips = 448 miles & 19.5 gallons x 22.5mpg = 438 miles per tank).
About a month ago, (maybe 6 weeks), my fuel economy plummeted back down to 3 round trips before being told I need to fill up and I don't know why. It did not seem to be a gradual decline either; I'm pretty sure all of a sudden, I was being told to fill up at the end of my 3rd commute. This is a difference of about 112 miles, though obviously none of the gauges are exact, so maybe 80 - 110 miles different, regardless it's a large decline and the fact that I am getting low fuel after only 3 round trips shows the numbers (1 trip = 112 miles lost).
Some additional details:
I used to use BJ's gas and I have been told there is no detergent in it. Recently, I switched to either filling up at Sheetz or a Shell.
I live in the Northern Virginia area.
I have noticed over the last 5-6 months, I will get to work and park and the digital readout will tell me I have 100 miles left (for example) and at the end of the day when I turn on the car to drive home, it will say I have 50 miles left (again, an example). The immediate changes in the digital read out are both drastic and sudden.
Sometimes when I fill up, the digital readout will tell me I have 380 miles in the car and other times, it will say 450 miles.
There has always been a discrepancy between the digital read-out (how many miles I can drive with the remaining gas) and the fuel gauge (how much gas is left in the tank), though it seems to have become an even wider discrepancy lately. I've noticed that the digital will read 20 - 30 miles left while the fuel gauge will show a quarter of a tank. Based on those numbers, that disparity is roughly 80 - 100 miles (1 tank = 448 miles --> 1/4 tank = 112 miles and Low Fuel = 20/30 miles), which is almost exactly the number of miles I am losing.
So, I brought it back to the dealership I purchased from (a franchised Nissan dealership) and they're telling me that after examining the electronics and the technician taking it home and putting about 100 miles on it, they can't find anything wrong, and the car is in perfect working order.
My instinct is that they're simply doing what the "handbook" tells them to check and not putting any further effort into it, but I obviously have no idea. I have / had other issues with that dealership concerning their attention to detail on other (unrelated) items such as the fluids, etc. but while I'm really uninformed about the technical aspect of all this, I'm having a hard time just simply believing that while I am getting 25% less fuel economy, everything is A-OK. I feel like, if the car wasn't under warranty, they'd be telling me something is really wrong....or it's something other than what the Nissan requirements are for maintenance and checks...
Can anyone give me their thoughts on what the potential issues are? What should I do about the dealership telling me there is nothing wrong? Is a 25% reduction in fuel economy normal? I'd never paid much attention to my mileage until I got this job, drive a lot and spend a crap-ton on gas. Now, I watch it like a hawk to try and maximize the fuel.
Any help would be appreciated.