MPG calculator reporting incorrectly? What is going on?

^^ Yep
The electric system can actually put out up to 20 kw of power, but this is a max usually only seen if the traction battery is charged to the gills or the driver has mashed the go pedal. For most day to day driving conditions and battery states the ICE will kick in at much lower power demands.

It is difficult for a new owner to know what is normal function in the Prius and what implies a problem. A noticeable drop in MPG *can* be a sign of battery problems, but it is not an isolated finding and an old 12V battery is a lot more common than a failing traction battery as the cause. The tell-tale additional sign is a fluctuating traction battery SOC where it used to be pretty stable in the past in the same conditions.

A driver who *tries* to EV, e.g. for enjoyment or out of a mistaken notion of gaining better fuel economy, is going to notice a lot more occurrences of this type. The car is suggesting that the driver restrain him/herself

 

Yes Jimmy, that's exactly how it happens. The fuel volume that you use to calculate the MPG isn't necessarily the fuel volume that you actually used, and it will vary with the vagaries of the fuel bladder and the pump cut-out point.

When I fill the Prius I'm pretty good at getting a consistent pump cutout and my calculated MPG tends to be fairly consistent (mind you we don't have the fuel bladder to contend with in the Australian Prius so it's easier to do). When my wife fills however then it's basically just where-ever it cuts out, hopefully somewhere near full. So I get exactly the phenomenon you describe. One tank we will have driven say 1000km and I know it's used at least 42L but the wife will tell me it only took 34L when she filled. I'll know it wasn't properly filled, but if I use that data anyway then I calculate close to 70 MPG or something outrageous. Then the next tank it only goes 750km, no surprise because it was never filled properly. So then I refill correctly and it takes 43L, so this next tanks comes out something like 40 MPG.

That's why you have to take the long terms averages. BTW, what does your prius displayed mpg read?

 

Ok so numerically, what's that. About 40 - 45?

What I'm really asking, is the displayed MPG fairly consistent despite the tank to tank variability in the hand calculations. Most people report the displayed MPG a couple of percent optimistic, though personally I find mine pretty accurate.

In any case it sounds like you're tracking somewhere about high 30's right, which is definitely on the low end of the range as you said. Tx, so this is with pretty heavy A/C use right, might be interesting to see what happens as the weather cools.

BTW. You said your tires were "stock", can you check the actual size and brand-model (eg Goodyear - fuelmax, 185/15).