Better economy at the top of the tank?

I believe there are several factors here:
- Small sample size for mileage right after filling up and reset. I'm assuming you reset your trip computer after filling up.
- You've presumably driven to the gas station the engine's already warm and still warm after filling up. Warmer/warmed up engine == better mileage.
- Subsequent drives after the car's been parked overnight/many hours == cold engine and mileage hit from that vs. the warm engine I mentioned earlier.

As for bolded, I doubt it. I highly doubt there's any sort of force feedback mechanism in the pedal. Toyota's never done that for their other Priuses, AFAIK.

 

I suspect if the OP left his car at the gas station overnight (so that the engine cools down completely), after filling up, he wouldn't see what he's reporting.

 

There's no obvious explanation for what you're perceiving.
The only general trend should be that the car is losing weight with the consumption of the gasoline which should improve the performance and the fuel mileage.

 

It's not going to improve in a way that the OP is feeling it.

 

Perhaps it's outside temperature dropping. Has this happened multiple fill-ups?

 

The Prius Princess and the Pea.

 

Particularly since the OP says that it loses power and mileage as the tank gets consumed.

 

With over 50,000 miles over two Prii, and resetting one of the Trip gauges daily for rapid MPG feedback, I must say 'No'.

The only thing I can posit is that if the refill happens after the engine is (mostly) warmed up, and the Trip MPG reset at that point, the rest of that particular trip will have higher MPG than normal because it does not include the usual warmup penalty. But subsequent trips won't have that benefit, so the tank MPG should start declining asymptotically towards normal.

No clue on the acceleration observation.

 

I am actually with the OP on this, sort of.
I think of it as fluctuation in the display reading.
Sometimes we are driving along consistently at 50-52 MPG for several hundred miles.
Then we stop to get gaso for 10-minutes on the highway, and after that just getting 45-46 MPG consistently.

 

The problem is that he's describing the opposite. His mileage is (apparently) getting worse until he fills up--then it's better.

 

^^^I did say "sort of "...I am suggesting maybe display reading can be impacted by unkown factors. I am suggesting this is display reading issue not actual fuel MPG calcs. This is a curiosity for me, I never reported here since I did not have enough data to confirm my perception.

OP could do real calc MPG from his fuel fill-up data to check his perception. and of course I don't have a c, just a thought.

 

Hi moofur,

From the observations of my C, the MFD does show big variances (usually better) FE after I reset the trip meter for the 1st 200 km or so. After that, the variances would become smaller as it approaches the bottom. I set up the other trip meter to measure lifetime FE which I 've never reset and there is not much difference whether it is near the top or bottom of the tank.

As you can see, it is not that the car is more efficient near the top of the tank. It is because the way FE is calculated. Having a small no. (distance) divided by another small no. (fuel consumed) would give more errors than a big no. division.

Can you try not to reset the trip meter or use another trip meter for your next fillup to see any difference?

Vincent

 

Since I too have been resetting my trip meter everyday for years and have never seen that pattern, I too doubt the observation.

Knowing how much MPG can vary from day to day and realizing it takes a large sample to confirm results, do you have enough numbers available to share with us yet?

It's far more likely to be coincidence. That does happen. Driving changes from tank to tank anyway.

 

I have noticed it too, no explanation though

 

I think we need more data here. If you are making a consistent commute every day, can you post the mileage, temperature, gas tank level, wind, and any other data you think is relevant, for each day? If you have Torque or a Scanguage that might also prove helpful.

 

We've had 2 hybrids from Toyota so far and we noticed poorer indicated mileage right after a fill up on both. MPG eventually moves back up to normally indicated levels after 25 to 50 miles of driving.

On the Gen II, after a fill up, we would see 47-48 MPG... that would eventually resolve to 51 MPG or so.

On the Gen III, after a fill up, it starts out around 49-50 MPG and gets back to 52-55 MPG depending on type of trip.

Just as a matter of interest, at the end of a tank of gasoline... 10 gallons of gasoline weighs about 61 pounds (on average)... so by the time you are down to 1 gallon, you've burned off 55 to 56 pounds. A lighter car should get you slightly better MPG in most situations. Maybe the Prius c benefits from greater weight allowing better glide?