Extremely low MPG, could the battery be going out?

pleasure.

 

Weight alone can't make that much difference. Something else (speed? temperature? seasonal fuel blend? wet road? wind speed / direction?) was different too.

My crude rule of thumb, created for a previous car but still somewhat applicable to my Prius, is that a cold engine start costs about the same amount of fuel as driving two miles on a warmed up engine. This is extra fuel required just for engine warmup, in addition the fuel needed to actually move the car.

By this rule, a 3 mile trip from a cold engine costs 5 miles of fuel, knocking a 50 mpg car down to just 30 mpg. Prius sometimes does a bit better simply because it can complete some shorter trips before its engine is fully warmed, thus not consuming the entire warmup penalty.

The way to avoid this warm-up penalty is to use a real electric car ...

... or bike, or walk.

 

Block heater helps too.

 

No vehicle does particularly well on short trips.
It's all relative.
If it makes you feel better 34 mpg, would be upper end for mixed driving for me, with my full ICE Honda Fit.
So comparatively your probably getting excellent gas mileage compared to nearly anything else you would be driving in the same short trip scenario.

 

Aren't you referring to ICE-driven cars only, those requiring warmup of a large iron-aluminum mass?

Pure electric cars, and plug-in hybrids that can do the particular trips on pure EV, should do just as well on short trips as long.

 

Better, actually, for PHEVs.

 

I stand corrected.
Yes, pure electric EV's...and plug in's can offer consistency in short trip performance.
I was pretty much limiting my thinking to Hybrids ( Maybe because that IS what the OP owns, and NOT a Plug In or EV ) ...and ICE vehicles. My mistake.