i had the dealership put in a new battery today.
upon leaving the dealership, i noticed the MPG display was frequently changing.
the fuel tank is 3/4th empty.... and the MPG display should barely move with a near empty tank. but on the drive home, the monitor frequently changed...... up and down, generally within a 1.5 mpg range.

when i arrived at the dealership to get the battery and service, the display read 33.0 mpg.

driving home it ranged from 29.7 to 32.0
as i continued to drive it seemed to settle into a range between 30.7 and 31.7 mpg..... but was frequently changing.

i took it back to the dealer and they said they dont know why that's happening.

any ideas would be appreciated.

 

maybe because since u change battery ecu reset so its relearning its self so should go bak to normal

 

I guess you don't know that the display goes by average distance. When they disconnected the battery the distance reset. Most people reset there trip counter after every fuel up.

 

^^ that to lol i never reset mines lol

 

thanks!..... yeah, that makes sense. i'm stupid for not realizing that.

i do reset it every time i fill the tank..... and it's acting exactly like it was just reset..... because, as you rationally pointed out, it was reset when the battery was disconnected. doh!

 

 

Glad someone else answered this. haha.

When I test drove the car it took me FOREVER to figure out how to even reset the avg mpg. The salesman had no idea and I got pretty frustrated with the car before I realized it was tied to the trip meter.

 

you should be able to tell if it was reset by looking at the trip counter. if you live 10 miles away from the dealer and there's 10 miles on the trip counter... that's what happened. if it has 100 miles on the trip counter but you only live 10 away... something else occurred.

~SB

 

no..... the trip counter was not reset. everything was exactly as i left it before they put in the new battery. so the trip counter had roughly 260 miles on it.

the MPG monitor was certainly exactly like the trip meter had been reset.

this is all confusing my easily confused mind.

 

Something is in an changed state due to disconnecting the battery. I'd zero the trip miles so the avg MPG is reset and go from there.

 

As others posted changing the battery also reset the mileage increment to zero. Most mpg systems change fairly often after the mileage resets.
Have you noticed the mpg changing often after you refilled the tank? Mine does. On my hybrid, the mpg is instantaneous so I can adjust driving accordingly but once past a quarter tank's worth of fuel consumed the average stays pretty constant.

 

The MPG display shows the average miles per gallon since the trip odometer was reset. If the battery was changed, the loss of power reset the trip odometer automatically. Not necessarily the one you see in-dash, but the one used by the MPG calculation routine, which is probably RAM, not ROM.

I notice the same thing whenever I stop for fuel and manually reset the trip odometer. The MPG display fluctuates rather quickly over the first few miles (especially in town, with stops, starts, and sitting idle at red lights), but then settles down as I rack up more miles.

For my first road trip, where I went nearly 1500 miles round-trip, I didn't rest the trip odometer once. It was rock-steady by the end of the trip, because a small fluctuation over a fraction of a mile counted for nothing against an elapsed 1500 miles. Checking the MPG readout against manually calculated mileage showed 35.7 on the readout, vs. 35.68 hand-calculated. That tells me the onboard display is accurate within the limits of round-off errors.

 

gooood info! thanks!... because the trip odometer didnt read zero, so that confused me.

 

Can't accept your thanks. That was conjecture, not proven fact. Should have been more explicit about my guesswork.