My displayed MPG does not make any sense

Welcome to hybrid ownership.

Getting higher MPG going uphill wouldn't make sense if you were driving a conventional vehicle, but hybrids change some rules. It's important to keep in mind that the MPG figure represents the instantaneous energy consumption of the ICE, not the energy consumed by the entire system (ICE + motors).

In the case you cited, the electric motors were doing quite a bit of the work, resulting in a net decrease in energy stored in the traction battery. Essentially, the hybrid vehicle computer did the math and determined that it was most efficient to use stored electricity to climb the hill. The ICE was put into a high-efficiency power band, and the result was 48 MPG while climbing a hill.

28-35 MPG on a flat stretch, even at 70 MPH, is rather low. Chances are one or more of the following was influencing the low mileage:

* The "flat" stretch was actually a long, regular incline. You could always measure it with an inclinometer for fun.

* There was a moderate headwind. Drag increases as an exponential function of air speed. Rule of thumb is, the faster you go, the lower your mileage will be in ANY vehicle. At air speeds above 60, the mileage really drops off quickly.

* Any number of other factors covered in the driving tips FAQ, including underinflated tires, low battery, &c., &c.

 

A quartering 20 m.p.h. tail wind for most of a recent 125 mile trip to Southern Ca at the prevailing (70 m.p.h.) freeway speed gave me a displayed average m.p.g. of 46.7. The route includes many slight inclines and two 7% grades several miles in length. I refilled the fuel tank at a great discount station, resetting the trip meter and average counter. Returning by the same route the next day after dry frontal passage so I now had a quartering 20 m.p.h. head wind gave me 45.5 m.p.g. The exact wind direction is hard to be sure of, but several flags acted as windsocks along the way, and I judged the direction to be c. 30 degrees off the bow both ways.

The Prius has a pretty slippery profile, I'd say.

 

One other thing that I have observed is that the instantaneous m.p.g. is the best inclinometer that you can buy. Often I think I am on a flat stretch of road and the M.P.G. shows me I am not. I no longer trust my eyes!

 

a couple things could be at play here... but only one thing is obvious...

the only way you will be in the 28-35 mpg range is if you are on a very steep hill or the car is not warmed up and i suspect the latter. on the return trip through the same area even if going up hill if the car is warmed up, the mileage will be much higher.