MPG calculator reporting incorrectly? What is going on?

I'll answer your question, and then I'll answer your question.
Disregard the MDF if you're interested in knowing what your mileage really is. It's going to be off by about 5-percent, and then you're only looking at what you're measuring with the display. Some folks go tank to tank. Some go trip to trip.
Not all trips and tanks are the same.
OK...so the first step is to make all tanks the same.
You can actually do this mostly easily and fairly accurately. My methodology is for a G3 (10-) but your G2 ought to work out about the same. Fill your tank regularly with x.xxx gallons of fuel. Actually it would really probably be x.xx, despite the fact that modern fuel pumps are really pretty accurate, but close enough is close enough.
For my G3 the math is pretty simple. 11.9 gallon tank. I drive until I reach DTE-50...meaning DTE=0 and then fifty more miles after that. I'm almost always able to pump 10.000 gallons of fuel into my car
Makes for easy math, since like most people I have 10 digits to help me with the counting.
The key is consistency.
Then...you'll KNOW what you're getting tank to tank, and how goofy the MDF REALLY is.

So that's the answer to the question that you ASKED.
Here are some answers to the question you were asking about.

You need to do a few things.
1. Probably replace your Aux battery. The readings look a little suspicious. Did you remove the battery to test it? Did you test it under a load?
2. Set your HVAC to 75. It's 95-99 here as well, and I guarantee you it's at least as humid. I set mine to 75, and at night I actually have to move it up to 78. If you (or your wife) have it on 68 then something needs looking into, and I hope it's the HVAC. If your dear wife is going through menopause, give her my sympathy and be patient with her WRT HVAC settings.
If she otherwise "has" to have HVAC settings <69, then give her a gym membership.
Please understand me....I don't care what you set it to, but you asked. Most of what an A/C does to make you comfy is actually dehumidification. On short trips, you're just going to have to live with a higher HVAC ding in your economy. Setting it on 60 doesn't cool the car faster than setting it on 70 (given a 95-99 degree ambient temp) it just runs longer.
3. If you don't know what pressures your tires are inflated to, then you need to find out. If you see a Mk-1, Mod-0 "pencil" type gauge anywhere in your house, throw it away. Spend about $8 and get yourself a good dial type (round) gauge...and USE it. Digital ones are OK I guess...but dial gauges don't require batteries.
Consider bringing your tire pressures up a little. If you're not checking your tire pressures every tank, then you deserve to get bad mileages b/c getting good mileages just isn't the top turd on your pile right now.
4. Oil...Tires...Same thing. If you don't know if your oil is overfilled---then you're not checking your oil. What KIND of oil are you using? If you had it changed....where did you do this? If you took it to an independent oil replaceorium, what did you tell them to use??? What did they really use?
Your tires are four years old if they're "factory"...and you bought the car used---so they may not be OEM tires.You need to know what brand and size they are.
5. Cargo. What's in your car? The G3 only has a cargo capacity of about 800#...pax included. Are you hauling a ton of extra crap with you? Do you carry a couple of Mastiffs in the cargo bay?
6. Your car is used. Do you know it's pedigree? Has it ever been wrecked? How sure of a "No" answer are you really??? It's a lot like love and marriage. You're married to it now, so it doesn't matter but erratic behavior now may be indicative of an undisclosed and very wild upbringing.

Some of this may sound a little pointed, and I'm not trying to be snarky. If you want to be efficient at something you have to work at it a little. Just buying a Pruis? It will get you 'good' but not really REALLY good. I know folks who drive well maintained, single owner babied G3's that get around 40mpg. They drive how they drive, and 40 is good enough.
I also know people who get closer to 60.
Some things you can't control. Pedigree, weather, geography, etc... This is why, if you're into the hypermiling life, or if you want to be green ($) or green like Kermit you have to CONTROL the things you can.
It's really not that hard, and believe ME you don't have to be that anal.
It's simply a matter of checking your tire pressures and your oil about every tank or even every other tank. Know your maintenance schedule and find out the status of all of your filters and fluids....for instance....how often do you replace the air filter? When was it last done?
Just a little extra (and consistent!) effort here will pay off and then you'll quickly vault to the upper five percentile in automotive preventative maintenance!

Get cracking and report back...-or- just put the giveacrap switch in the "don't" position and get on with your life.
Your call.
I know people who would be dancing in the streets if they got 35mpg!!

 

OP, zero the tank MPG meter and go for a 20 mile round trip drive on the highway at 60 mph WITHOUT AC in the cool(er) evening. Fuel economy below 50 mpg suggests the car has a problem.

 

I'm sorry if I hurt your feelings, and I did read my post before I hit the send button. Twice.
I don't like (unintentional) grammatical errors and a forum is a poor place to convey information since there is no sarcasm font.
There were a lotta "I do not know's" in your first posts, and that led me to poke you in a sensitive place to get you motivated to find out. Your reported mileages are well below the norm for a G2, which even if driven rather spiritedly should give results closer to 43 than 33.
This means something is wrong.
Whether it's a dead short between the seat and the steering wheel, or there's a mechanical problem with the car is something I'd be finding out---ESPECIALLY if you haul young children around.

We've established that you have OEM tires of some pressure, probably safe. I don't recall if G2's have TPMS (This system was mandated for passenger cars by my beloved government to thwart the beneficial effects of natural selection) but I think that they do.
You said that you had a "trusted" mechanic change your oil. What viscosity? My company has "trusted" mechanics that changed 2 Priuses in our fleet using the "right" oil as set forth in our company's repair and maintenance procedures one time.
They used 20w50.
OK...so. Lets stipulate that your wrench is da bomb!!
What viscosity oil did he/she use?
Did they/you/somebody look at the air filter?
If your tires have 37k on them and you don't have funny wear patterns, then you're probably OK WRT alignment, but if you continue to get MPG's in the 30's I'd look into this as well after I checked everything else first.
Look at the underside of the car.
See all those plastic panels?
Are they all there?
Do you see any signs of a ramp strike, curb collision, or something else?
The previous owner could have had a low order collision with a curb that has not yet begun to manifest itself in improper tire wear.
Does your car track normally when you let go of the wheel at speed?


Like I said.
Something is up.
If you're getting 33 under the conditions that you reported in the survey, that something is either mechanical, or human. Hell....it could be that the pump at your local station is off kilter. Do you buy your gas at the same place all the time?

Good Luck!

 

Going from "32 all around" to "42/40" should make for quite a positive change in the mpg range you've been experiencing; ESPECIALLY if it turns out there's nothing mechanically / electrically "wrong" with your vehicle. I'd suggest checking the pressure about once-per-week for a month or so to "check" for slow leaks, and at least once per month thereafter.

For what it's worth, I've been running 42/40 for about two-and-a-half years ( we bought our '08 new nearly 4 years ago ) and very rarely have full tank mpg figures ( I usually fill up after 325 miles or so; with 2 or 3 "pips" remaining ) go below 50 during the warm / hot driving months; about 47.5 has been the lowest, and somewhat above 55 is not terribly uncommon. As always ... YMMV ... snicker ...

EDIT / an afterthought: I sometimes have what I refer to as "fluke" mpg results for a single fill-up ... that's when the readout mpg varies +/- 10% from the calculated mpg. ALMOST ALWAYS, by the time I fill up the NEXT time, the TWO fill-ups ( combined ) end up 2-5 mpg ABOVE the car's current, lifetime mpg.

 

Referring to your most recent update: 47 mpg vs. 37-40 ... that's REALLY encouraging! Rarely is only ONE "thing" the culprit, but we can always hope!

 

Good one Jimmy...I saw a good improvement back to normal MPG after Optima install but not for the first tank or two. Give it a chance to reset the settings and all.

 

Hey another good one Jimmy ...that's what I saw too! Green green green. Now in the summer we are in the blue but we had a good 6 months in the green during the winter.

 

As strange as it may sound, you really don't want to see all 8 bars of (traction) battery charge too often Jimmy. The "nominal" battery level that the Prius prefers to maintain is only 6 bars. This is because it needs to keep some capacity in reserve for regeneration. Actually seeing 8 bars too often can be a sign of something bad rather than a good thing. If however you saw it after regenerating down a reasonably long or steep hill then it is perfectly normal.

Given your 12V battery measurements I personally doubt whether the battery swap played any role in your mpg improvements. It was definitely getting a little weak so it was probably still worth replacing, but I think your improvements are just down to the tires and getting used to some of the quirks of driving a Prius. Also, given the inconsistencies of the Prius tank bladder, I'd give virtually no credence at all to the one slightly lower than expected MPG calculation based on a single 8.5 gal fill.

 

It has been 2 1/2 weeks since last update. All still going well? Tank MPG still in the high 40's?

 

Suggestion

 

Same crap that happens to my Prius. I park it with full bars, next day I get in and it has already drained some and then quickly drains to 2 red bars. I have no idea what causes this as my Preferred Hybrid Mech. said evertything was fine.

My ICE runs now even when costing/going really slow with no acceleration etc. when it shouldn't and didn't used to do so.

I put 44/42 in tires and get below 40 (32-38 MPG) per tank when I used to get 46-50 all the time.

Something is WRONG!

 

Maybe I'm reading the above wrong, but:

The motor assists when the engine is running as well. Electric-only propulsion tends to be under very light load conditions at around town speeds, by design. It will do a little better say in a parking lot at near walking pace, ie: allow gentle acceleration or hang in through a bit of upgrade.

And feathering the gas purposely to extend electric only propulsion should be done prudently: once the battery charge starts falling significantly you might want to jab the pedal, to purposely start the engine.