Odd mpg figures just started

[58 DeSoto]

2013 SE Hybrid 60k never a problem. My lifetime mpg was at 49.5 .  It slowly started.  To drop and went to 43 over about 2 years.  Then suddenly it dropped from 43 to 38 . Before it took a month to get it to move 1 tenth of a mpg. Now the lifetime mpg changes like the instant mpg .  It will go up and down 5 mpg in a 10 mile trip.  Also the mpg will drop 5 mpg from sitting overnight.    I looked for a fuel leak and haven't found anything.  I'm a car guy and have always fixed my own cars. Still has the original battery.  Car runs just like it always has.  Just the jumping mpg is strange 

[MeeLee]

More than likely it's a battery issue, if your mpg numbers get reset. Could be a low voltage issue due to the cold, should be gone when the temperatures increase., but maybe an indicator of a battery failing soon.

[58 DeSoto]

Don't think cold temperature is the problem as I live south of Tampa. The numbers didn't get reset just make wild mpg swings over a short distance trip. 

Edited January 21 by 58 DeSoto

[Waldo]

What's your actual mpg calculated from the number of gallons you put in the tank?  Basically we need to understand if the issue is that your actual mileage has changed, or if there's a glitch in the display in the cluster.

[58 DeSoto]

I'll have to do a actual mpg check next time I fill the tank.  That won't happen until next month as I don't do a lot of driving.  I'll post it when I get the numbers 

[MeeLee]

On 1/21/2024 at 4:49 PM, 58 DeSoto said:

Don't think cold temperature is the problem as I live south of Tampa. The numbers didn't get reset just make wild mpg swings over a short distance trip. 

If your (12V) battery is over 4 years old, yes, then even the 50s or 60s can cause this, as lead acid battery voltage and current flow, highly depends on temperature.

The lower the temps, the lower the voltage.

If your voltage drops below 11.8V, all kinds of glitches happen.