Average MPG: Reset or Let it Ride?

[johnny51981]

Do you reset the overall Average MPG after every fill up, or do you let it ride?

[DonDNH]

Let it ride...

[auricchio]

I just let it ride because it isn't really as useful as my lifetime-MPG spreadsheet.

Occasionally, I might reset it just to see what I average for a specific trip.

[Rancid13]

I used to reset it at every fill-up to try to get a higher mpg on the new tank, but after several tanks of sub-par mileage (44-45 mpg) I got a 48 or 49 mpg tank and haven't touched it since. That was sometime in early fall, and my mpg since then has been between 47.1-47.3...it varies very little nowadays.

[paprius4030]

I just let it ride because I don't want to get depressed when I see what my winter mpg's are LOL. Letting it ride gives me a better number since as the car gets more and more miles it takes alot to make it change from the 47.7 lifetime avg. I got now.

[Danny Hamilton]

I use a spreadsheet to track my history and lifetime average MPG. This way I don't lose the information if the car ever needs repair that resets the MFD.

I use the MFD to keep track of how I'm doing on fuel economy for the current tank of gas so far. It works great to motivate me to keep trying to do better. The only way for that to work is to reset after every fillup. Since the miles resets anyhow, it makes sense to reset the MPG.

Also since the gas guage doesn't reliably indicate how many gallons are currently in the tank, I can use some quick math:

(gallons pumped at last fill-up) - (miles traveled since fill-up) / (MPG for this tank) = approximate gallons of fuel until next fill up

to figure out how much fuel I've got. Additionally I can do some more quick math:

(approx. gal. of fuel until next fill-up) X (estimated MPG for upcoming trips) = miles until next fill-up.

This can help me decide if I should stand out in that cold wind to fill my tank now to avoid needing to fill-up during the blizzard predicted for 2 days from now, or if I should put off filling up during the rain, because I can easily make it to the warmer dry weather in a day or two.

This math doesn't work if you "let it ride":

9 gallons pumped at last fill-up with 10,000 miles since last MFD reset and 54.5 MPG displayed.
350 miles driven since last fill-up and MFD now shows 53.8

If I do the math I get 350 / 53.8 = 6.5 gallons used
9 gallons pumped - 6.5 gallons used = 2.5 gallons until next fill-up
estimated 50 MPG for next few trips = 150 miles until next fill-up

Had I reset at the last fill-up, I'd have seen:
350 miles driven since last fill-up and MFD would show 40 MPG.
350 / 40 = 8.75 gallons used
9 gallons pumped - 8.75 gallons used = 0.25 gallons until next fill-up
estimated 40 MPG for next few trips = 10 miles until next fill-up

That's an error in estimated miles left of 140 miles!

[ryanschram]

for me, it doesn't matter when i put gas in, it always resets

[DaveinOlyWA]

i always reset it because i want to know how temperatures, driving conditions and even how different drivers affect the mileage.

also, a sudden unexpected drop in mileage tells me something is wrong because i KNOW what to expect since i have tracked each tank individually. after nearly 60,000 miles on two different Priuses, there is little surprise for me.

recently my SO because the primary driver of my Prius and because of her higher comfort needs, our average has dropped 5 mpg!!!

all because, i rarely use heat, set at 68 if i feel the need. only enough defrost to keep the windows clear. etc.

she does not want to be bothered with toggling the cabin climate controls like i do, so when i get into the car, the temp is usually set at 80. defrost blasting away, etc...

so i guess this should be in one of those "why does my mileage suck?" threads... but

[Doc Willie]

I ofter reset it each trip. I want to know how I am doing on my hypermiling technique.

[Rangerdavid]

yeah, i reset every time. i try to improve every tank.