I'd like to edit the FAQ to include "Why did my mileage drop?"
I have a short list (which may or may not be correct)

• What if driver, inadvertently reset the mileage counter amidst the trip, thus recording fewer miles per tank than actually driven?
• What if the recorded amount of gas used was mistaken. What if driver purchased 13.5 gallons, but recorded 15.3?
• What if the tank before the trip wasn't completely filled (fuel shut off early) and then the tank after the trip was filled to the rim, thus recording far more fuel for the one trip than actual.
• What if there was a significant difference in the factors relating to the cars performance in this one tank (or trip)? For example, if driver, weighing 140 pounds, typically drives alone with an empty trunk. What if driver traveled with four passengers each weighing 250 pounds and the trunk was full, adding an additional 250 pounds. In this situation, the extra 1250 pounds will cause the mileage to suffer. *
• What if driver typically purchases gas without ethanol, but for this trip, got 10% ethanol for this tank?
• What if tires were under inflated?
• What if there was a head wind during the trip?
• What if trip included a few stops, whereas driver typically drives long stretches without stopping?

* Supposedly, each 100 pounds reduces mileage by 2%, so could 1250 extra pounds reduce it by 25%?


Please reply to this thread with your suggestions, comments, etc.
When we have a better list, I'll have admins open the FAQ and I'll pop the answer in.
Thanks!

 

Got one to add to your list.

What if wife was sole driver of the car for a whole tank and her normal driving habits are "pettle to the metal immediatly followed by severe braking"

The last two tanks have dropped from around 34 to 29. A second factor might be the heat - it's been in the high 90's for the last month or so.

 

The question comes down to: "Exactly how smart is the fuel system computer?" If it's smart enough, it should be able to determine the amount of fuel put into the tank, and not just assume a full fill when the guage goes up. If the tripmeter is reset in the middle of a tank, it should simply restart the whole calculation from the new set point.



KenSoren:
The increase in temperature is known to cause a decrease in mileage, due to the greater energy required for the climate control system. There is also a loss of about 1 mpg per every 15 degrees F above 60 F due to heat related expansion of the fuel itself.

High 90's??, High 90's??? We go for weeks here where it doesn't get DOWN to the high 90's.

 

I have reset the fuel system computer more than once between fill ups and agree that it does handle the calcs pretty accurately.

RadioTec - Got to say that living in Phoenix in the summer is quite an accomplishment. We stopped to visit my wife's sister for a couple of days in July many years ago and got exposed to the low temp of 90 degrees. It was so hot that the swimming pool felt like a sauana and we were glad to move on ASAP. What kind of mileage do you see in the heat. Ours dropped to just under 30 by the computer which should compute to an actual 29 or so.

Concerning the temps - we have been consistently in the 99 to 102 range with one day at 110 with a low of 79. We usually get down into the 60's in the evenings and we depend on the cool evenings to cool the second floor bedrooms off. The 79 degrees makes for a miserable night, so we were glad to see that night pass. Tonight so far it is 62 degrees - good sleeping weather!

 

Actually, I was referring to the trip odometer, which would cause a miscalculation, if one calculates it manually.
The new reading for miles would now be incorrect when dividing by the fuel used.
I rarely calculate it manually, however, since I know the onboard display is usually within 1-1.5 MPG of 'reality' (and life's way too short for me to care whether I got 38.4 or 37.1 MPG on a trip).
At fill ups, I just glance and see the number and as long as it's in that range, I'm okay with it.
As to the 'smarts' of the system you asked about, I believe (and am usually wrong) the car figures out the fuel at fill up by the gizmo in the tank that reads the fuel level. I'm guessing here, but assume that when the fuel door is open, it resets the system and then inputs the beginning fuel level by some type of a reading (it might think 'the tank has 15.5 gallons in it' by virtue of the float mechanism). As the tank is depleted, the float records the fuel level and calculates the MPG (or LP100KM if you're up north). The 'error' IMHO comes into play when the car is first filled up. I believe (and again, am usually wrong) that the amount of fuel at fill up is overstated as the process of fueling adds a lot of air bubbles and causes the volume to appear higher than it is. I'm assuming the car thinks you have 15 gallons even though you just purchased 14.8 gallons. This error in beginning value sets the stage for a series of miscalculations over the life of the tank. When the tank is near empty, the extra 'air bubble' volume (assuming that the gas sloshing around while driving to the station picks up bubbles and, similarly, overstates the volume) is negligible since if you add in 5% 'air' to .75 gallon it's a lot less than 5% 'air' in a 15 gallon fill up.
I fully expect an engineer or physicist to reply with a twenty paragraph reply shooting holes in my theory, but that's why I think the onboard display is always wrong. If you do debunk my theory, please keep it simple...I got straight A's in math and physics, but that was over 30 years ago!

 

I'm reminded of the very old saying:

"The man with one clock always thinks he knows just what time it is. The man with 2 clocks is never quite sure."

Many of us have 3 or 4 clocks, here:
Actual fill-up gallons and odometer
Tank average on the MFD
average on the Nav screen display
Scangauge data

None is ever exact. Each has it's own sources of error, and the error sources are all different. Each can be either high or low. It would be a miracle if they all agreed. Even if two were to agree exactly on some occasion, that would probably be more the result of coincidence than exactness.

There are many good uses for the mpg data. Probably the most common is "bragging rights." For this purpose, I recommend collecting all available data and bragging about the best.

 

I have a habit of logging my gas purchases in a book kept in the glove compartment. I think that actual miles traveled divided by gallons used is as close as you can get to actual MPG. I always fill the tank and include the date, total miles off the odomoter, gallons & dollars purchased, and of course eventually figure the MPG. The log also includes all oil changes, tire rotation & other maintenance performed which is handy to jog my memory when things were done.

I have always calced the MPG off the log just for my information - I figured if it I see it going south I need to have it checked out. I have done this for as long as I can remember - probably 30 plus years, so nothing to do with having a hybrid, probably just excessive compulsive record keeping.

 

I much prefer the readings of mpg from the mfd. (dash display) It doesn't depend on how level your car is when you fill the tank or if you short fill your tank and etc.

I use the mpg readout mainly to see how my gas mileage is doing as leave the house and when I get back home. It's quick to use when I refill the tank at the pump when other cars behind me waiting for gas.

Like haroldo says, I subtract 1 1/2 mpg to give me closer to a calculated figure. Looks like the mpg reading is at least more consistent from tank to tank. Could be that the fuel pump inputs the gallon information to the ECU to process the instant mpg reading.