2nd Tank of Gas

Why do you wait until the tank gets so low? Fill it up until the 1st shut off. Don't try to stuff the tank. We've all been there. Fill the car the same way each time to get an accurate comparison. I have found that after 2200 miles the computer is generally correct. I may stop taking manual readings. The last time my computer said 52 MPG and my manual reading came out to 66 MPG. You'll get the hang of it.

 

I've never been able to get 9.75 gallons in. I think your dealer over-filled it. If the computer gave you similar average mpg readings for the tank, then it's just a discrepancy on the fill amount.

Standard advice is do not over-fill!!! There is a vapor trap that can get gas in it if you over-fill, and then you will pollute more.

You can also get more miles per tank if you are willing to risk running out of gas. There seems to be some variability (sometimes a lot) in the amount of gas remaining when the last bar starts to blink. But running out of gas is not worth it just for the bragging rights of saying you got 500 miles.

 

Yes I agree, do not overfill the tank. Also the manual calculations are different for me on every tank. I usually do better per the MFD than I do by my manual calculations by about 2 or 3 mpgs. I too may quit taking manual calculations. For example if you fill up when it is 28 degrees F and the last pip is blinking you may add 8.5 gallons. Now you drive the car 440 miles down to the blinking last pip. Then you fill up again, but now the temp is 75 degrees F. The tank may now hold gallons 10.2 gallons. By the MFD you may have got 48 mpg but by the manual calculation you only got approximately 43 and thats if you assume their was no remaining gas in the tank. Taking manual calculations with this car is a waste of time. The best way I have found to do a manual calculation is to keep track of the amount of gallons I have added over the life of the car and then just dividing this number into the odometer reading. This is much less complicated than the doing it each tank and is far more accurate.

 

I don't do manual calculations. It has just never appealed to me. Seems too complicated I guess. I reset the mpg button when I fill up (and I don't try to top off at fill-up too) and rely on the mfd to give me decently accurate mpg info.

While it would be nice to get a 500+ or 600 mile tank, I'm fine with not reaching that level of achievement and earning those bragging rights, and I'm also fine with not running out of gas in an attempt to squeeze every last drop out of the tank.

 

I'm a little disappointed in my MPG on my first tank. Yes, yes, I know...there's a breaking in period. And yes, yes, I know...I live in a cold climate. And yes, yes, I know...my to/from work commute is short (3.5 miles each way).

But I'm down to two pips left on the gas gauge and I'm at 33.4 MPG on the MFD.

 

Well, there's also a breaking period for the driver. A learning curve, where the car will reteach you how to drive more efficiently...if you want to.

 

Yep. Been paying close attention and learning/adjusting.

 

I had the same experience. The tank from the dealer took me 540 miles, and the first tank I put in took me 480 miles. I am not going to worry about it. Both tanks average over 51 MPG, so I am happy. Now if the HOV stickers would arrive I would be all set!

 

No, do not overfill the tank. That won't help your mileage, just makes you look like you did better. The tank average on the MFD is very accurate because it bases its calculation on fuel used, not anything else. Doing a tank average based upon how many gallons you fill and how many miles you drive is not accurate becasue the fill amount will vary from tank to tank becasue of the varying sensitivity of the auto stop functionality of pumps. If you were able to refill the tank to the exact same level every time then you could use that kind of calculation to determine your FE.

Your car is new, it is tight. That is working against you. If your climate is cold and you have short commutes, that works against you also. You will have FE that is much less than the EPA falsetemates if any of these conditions are true. Most of us will never see the EPA numbers because our driving habits, will never be in line with the test procedures that the EPA specifies.

I'd say you are getting somewhere between 40 and 45 mpg. That is excellent. Enjoy what others aren't getting with their conventional vehicles, especially in the winter.

Now go fill your tank before you run out.

 

Go check out John1701a.com website. That guy tracks everything and he's proof that the MFD is not accurate. It's generous.

Since the gallons are so low (only 10 or 11 for a tank), I agree that any single tank measurement is a guess at best with a high mileage car.

Measuring all your tanks against your total odometer reading is the only way to be actual and accurate over time.

In a couple tanks, you're not that close, but after 10,000 miles, an extra half gallon doesn't impact realtime actual mpg. You'll make it up next time, or over time.

If you track your mfd against real gallons and the numbers are off by 2 or 3 mpg, which do you think is right? The actual gallons... vs.. actual miles..

 

If you are an actuary, just drive the car. I am an accountant and have already confused myself enough trying to analyze fuel economy. In fact, it is starting to change my thinking on things it shouldn't. For example, I had an Outback fully racked out before, and now I am negociating racks for the Prius on account of fuel economy. But now I have 58 inch load bars on her and get 50mpg still with two bikes on top. Now my screen stays on audio or climate as I know my sure footed coordinated right foot will do its job to maximize fuel economy. Don't forget that a lot of people on this forum let that screen dominate their driving experience. They don't even see through the windows anymore at the world around them. Only the road, is it clear....yes...hows my mpg....but wait it could be better.....rediculous. The Toyota engineers did more thinking than all of us combined on Prius Chat, so feel confident, Mr. Actuary, you made a wise decision and fuel economy will come with time....

 

410 isn't that bad. 500 is great in my book so far. I just filled up for the 6th time today and I've only ever gone 380 miles at most on a "tank". I got only 38 mpg by my calculations on my 1st tank (45.1 mpg on MFD). I'm not clear how full the dealer got it but all the bars were lit.

I don't wait till the last bar per the dealer's warnings. If it hits 2 or has been on 3 for a long time, I fill it.

My lifetime calculated average is 44.1 mpg so far.

 

Short trips kill mileage. So does extremely cold weather. You could live in Florida, where it's always hot, and be miserable, and only drive your Prius when you have a long trip, and use your old gas-guzzler for short drives and accumulate massive bragging rights over your lifetime MPG, or you could live your life in your own real world conditions, and enjoy the car for what it is.

Learn to drive it for good mileage, but don't worry if someone else does better in the long run, because s/he probably has different traffic conditions, weather, driving distance, etc.

I'm not going to switch from the grocery store 1.5 miles away, to the one 10 miles away, just to improve mpg. Nor am I going to switch from 3 times a week to once a month grocery shopping to reduce the number of short drives. That would improve my lifetime mpg, but that's not what life is about.

 

What Prius owner would be stupid/crazy enough to do such a thing?

 

The MFD is "actual gallons". The MFD uses fuel injection data and is accurate to within a few percent. Calculating MPGs for one tank using gas pump quantities assumes that the number of gallons you put in at the last fill equals the number of gallons burned since the previous fill. It should become apparent that the two are generally not equal.

Over many tanks, lifetime MPGs calculated using (1) gas pump quantities and (2) MFD values should approximately converge, ignoring any systematic errors in the most-often-used pump and in the MFD.

 

You've posted a similiar statement before. Is this based on what you think it should be (which makes sense), assumptions, or actual fact?

My 02 Nissan Maxima which I sold had trip computer that ALWAYS read 1-4 (usually 2-3.5) mpg too high compared to actual. Of the people I saw discussing the trip computer on maxima.org, I can't think of one person who said their results were different than mine.

My 350Z's is more accurate (usually 0-1 mpg too high; usually within 0.5). Both cars have the same engine and both are fuel injected.

 

I don't have a specific reference for the Prius; for a general reference on EFI see http://en.wikipedia.org/wiki/Fuel_injection. If Prius fuel injection were not accurate its emissions control would not be so good, because the fuel/air ratio would be chronically wrong. Of course even if the Prius EFI is highly accurate the MFD could be deliberating overstating MPGs to make owners feel better...

Note that gas stations have an economic interest in overstating the amount of gas shown as dispensed. This will make manual MPG calculations look worse than reality.

 

I agree that the MFD and hand calculation methods will converge over time. I'll just go with the MFD based upon what else has been put into the car's design and engineering. If it's off by one or two MPGs I'll still be happy with that.

Oh, I keep track of what I put in the tank and the mileage at each fill up. Unfortunately, it's just impossible to know how "full" the gas tank ever is when the fill up is done. Just like it's impossible to know how full the car was when the dealer filled it up. The only way to know for sure is to be able to actually see the fuel level in the tank or filler neck. AFAIK, that's only possible in a VW bug with a translucent, plastic tank.