new car mpg confusion

[SSimon]

i have an '06 prius. just picked it up a few weeks ago. i have 313 miles on the odometer and last night, topped off my tank with 5.043 gallons and it's my first tank fill up. (i still had just under half a tank of gas but had read that some station pumps don't work in the prius and wanted to make sure i didn't encounter trouble with little gas left in my tank...no trouble, i am happy to report). when i manually calculate my mpg's thus far, i come up with just over 62 mpg. my display tells me i'm averaging just over 42 mpg since i purchased the car. is my feeble brain not calculating the true mpg's right? why the disparity and why such great fuel economy from a new car?

[M. Oiseau]

Sounds like your prius was about 2.5 gallons below full when you drove off the dealer's lot.

[KTPhil]

The dealer might have put some miles and gas in the car without resetting the MFD trip odo. Look for an average over 2-3 tanks before drawing any conclusions.

[Marlin]

How many miles did your car have on it when you picked it up? Did you reset the consumption screen when you got your car, or did it already have some miles showing on it? The MPG calculation only resets after you add a certain number of gallons of gas to the tank (3 I think). So, here's what I think happened.

The dealer filled the tank when they received the car, then used it for test drives and put around 60 miles on it with 15 or so short trips, getting only about 25 mpg and using 2.4 gallons of gas. When you picked up the car, they topped off the tank, but didn't add enough to reset the consumption screen. You then drove the car an additional 250 miles or so (getting significantly better than 42 MPG) before filling it with 5 gallons. Since the consumption screen didn't reset when the dealer topped off the tank, the 313 miles traveled actually used 7.4 gallons of gas (your 5 gallons plus the 2.4 gallons they topped the tank off with), rather than the 5 gallons you put in. Thereby yielding an average 42 MPG.

Just a guess.

[SSimon]

thanks all.... my car only had 2 miles on it that the dealership said was a product of driving the car to fill the tank. the screen registered a full tank when i picked up the vehicle so i assumed that my calculation was correct when i refilled last night. it's probable, however, that the tank was not in fact full. i will perform manual calculations on the next two fill ups and if there continues to be a discrepancy, you surely will hear from me again. sorry if i've posted anything incorrectly or in wrong format, i'm a new user and have no clue about etiquette on this site. thanks for the help!

[jeneric]

Originally posted by SSimon@Mar 7 2006, 11:16 AM
...i have 313 miles on the odometer and last night, topped off my tank with 5.043 gallons and it's my first tank fill up...when i manually calculate my mpg's thus far, i come up with just over 62 mpg.  my display tells me i'm averaging just over 42 mpg ...

[snapback]221808[/snapback]

I would trust the MFD that you went 313 miles at 42 mpg and used up 7.5 gallons.

Because of the bladder in the tank or something else, you only got 5 gallons in the tank. The dealer was just able to somehow get 2.5 more gallons in the tank than you could at that moment. I would think it will all even out, but wouldn't get too excited about that 62 mpg number. Especially, because the next number when you are able to fit more gas in the tank will probably be in the 30's.

[SSimon]

it's me again! thought further about the dealership not putting in a full tank of gas but this would mean that i was able to get even MORE gas mileage than my manual calculations of 62 mpg.....means i would have driven 313 on three gallons of gas! not possible. hmff. since a dealership cannot legally set back odometers, there is no logical explanation for this disparity. we'll see how my next two manual calculations come out.

[DaveinOlyWA]

yep, first fillup means very little. your stats are so far off, i would be strongly tempted to discard the information as it will cloud several tanks in the future. (but save it just in case...might all work out on the very next tank!)

what you experience is just a difference in filling technique... dont suppose you remember where your mileage was when the first bar disappeared?

because of the bladder, my advice... devise a filling stategy and stick to it. be it rounding off after first click or packing in another 2 gallons. just be consistant and ignore the numbers until you have at least 4 tanks (record them of course!!) . reset your MFD on every fill up so you can compare from one tank to another, i think in 10,000 miles you will laugh at yourself for being so concerned over nothing.

look at my stats... they were averaging 3 mpg apart when i first got my car.

*edit*

had to look, my first tank, 49.1 MFD, 43.9 pump... not nearly as wide as yours...but...ya know

[jeneric]

I always put the pump on the lowest setting and leave it until it turns off.
I haven't seen a 2.5 gallons delta, but hit 1.4 gallons difference pretty easily.

[efusco]

Originally posted by SSimon@Mar 7 2006, 02:25 PM
it's me again!  thought further about the dealership not putting in a full tank of gas but this would mean that i was able to get even MORE gas mileage than my manual calculations of 62 mpg.....means i would have driven 313 on three gallons of gas!  not possible.  hmff.  since a dealership cannot legally set back odometers, there is no logical explanation for this disparity.  we'll see how my next two manual calculations come out.

[snapback]221841[/snapback]

So maybe the dealer topped off/overfilled and you underfilled..that would easily account for the discrepancy.

You're getting way too worked up about this only 300 miles into a tank.

Drive the current tank, fill when the last bar starts flashing, calculate your mileage for that tank and then average it with the first. Do that for 8 more tanks (10 total) and report back...betcha there's less than 2% discrepancy b/w your computer calculated average FE and your manually calculated average FE.