[CED]

Hi,
I live in Utah Valley in the state of Utah. My wife and I are in NE Oregon for a few months. We own a 2017 Touring and have liked it until recently when the mileage went down about 10 MPG in a week. We had just had an oil change and we thought it was the wrong oil in the pan. We did everything we could think of to get the mileage back, with no success (including having the folks change the oil again). We even had the dealer check it out when we went in for one of many recalls. They said there was nothing wrong.
A couple of days ago it was topped off with regular gas again. We were going to check mileage the old school way with paper and pencil. We looked at the amount of gas used and the number of miles we went and figured that we were probably getting the mileage we expected, but it was not registering that much. When I topped it off it took 2.1 gallons and we had gone about 90 miles. I think the car was listening to us because... within a few blocks it started registering mileage the way it had before... In the mid 40s and above. About a 10 gallon increase. What's going on here!!!!? I'm about convinced that there is an error in the dash mileage instrument panel reading. Both in the actual instant and trip mileage. The miles to empty seems to have never changed much. Any ideas???? We will keep watching it to see if it changes again. Maybe this is the reason so many see a drop in mileage.

 

[FlNiro]

My Prius would do the same thing. So far my Niro hasn't but I wouldn't be surprised if one day the same thing happens. There is a LOT of things that can change the fuel mileage. A little drop in temps. Driving up and down a lot of hills, did you check to see if they let some air out of your tires when they did the oil change? If you had more than the specs they may have lowered it to the specs. I generally run them at 40 psi and sometimes the techs. may lower them to 35. Mostly in my experience it's just a "mystery" and the mileage return to normal in a week or so by magic LOL.

 

[badyellowvette]

Similar thing happened to my 2017 EX once never happened again. Here is my post about my issue.

Fuel gauge weirdness.
Filled my tank the other day to check my mpg's vs the dash readout. When it was filled the miles left in the tank was 539 miles or something like that. After my drive 122 miles the miles left was down to 417, looked right. Mpg's on dash said 50.1 I did the math and got 53.3, I was happy with that. Problem was the fuel level gauge didn't move to full until I got home 10 miles and the miles left in tank is still on 417.
I worry about the post where mice or some critter chewed the wires going to the fuel tank. I haven't seen many mice in my traps lately and if that was the problem I guess it wouldn't work at all. Any ideas?

 

[deltasmith]

When I topped it off it took 2.1 gallons and we had gone about 90 miles. I think the car was listening to us because... within a few blocks it started registering mileage the way it had before... In the mid 40s and above. About a 10 gallon increase. What's going on here!!!!? I'm about convinced that there is an error in the dash mileage instrument panel reading. Both in the actual instant and trip mileage. The miles to empty seems to have never changed much. Any ideas???? We will keep watching it to see if it changes again. Maybe this is the reason so many see a drop in mileage.

Problem was the fuel level gauge didn't move to full until I got home 10 miles and the miles left in tank is still on 417.

Chapter 4 of the owner's manual mentions that you need to add at least 3 liters (when above 3/4 full) or 6 liters (in other situations) before the gauge will register the additional fuel. Sounds like you both added a bit more than that, but perhaps that is part of the explanation? There has to be logic somewhere that allows the gauge to give you a consistent reading, regardless of how much sloshing is going on in the fuel tank as you accelerate, go up hills and around corners, etc. It's probably doing some sort of time-based averaging of the signal received from the sending unit in the fuel tank.



My old Honda would update the estimated range based on the average fuel economy. If I reset the trip computer and started driving, my initial MPG in the first few miles would be reported as something like 16 (cold engine, city driving). As I continued to drive, if I drove in a way that increased the MPG, then the estimated range would also increase. The Niro might be doing something similar, but if it is, it's likely to be using some historical fuel economy data that isn't visible on the trip computer. For example, with the PHEV, if I reset the trip computer and drive in EV mode, it displays my fuel economy as 999 MPG, but it doesn't bump up my estimated range on gasoline to infinity.