MPG indicator question

[jgprather]

When should I reset the MPG indicator? I change it sometimes after I've just gotten gas. Do I need to reset it then? Or it will read, for example, 23.7, then when I reset it (not after having gotten gas), the number will be calculated completely different. I had the same situation in my MINI.:iono:

Any wisdom from others?

Thanks.

[Guildenstern]

It's basically giving you historical average. Including all time stopped at lights and Speeding off for that awsome 0-60 time. When you reset it, it starts from the second you reset it at whatever your instantanious MPG is, then averages from there.

I used to reset it with each tank of gas. But I found just doing the math came out more accurate and stopped doing it. Now I reset it whenever I try new routes to places I routinely go. Or when I want to see quickly how a current driving style is affecting my overall mileage. The Instantanious changes too quiclkly for good info, but does help you understand just how much gas you are using at that second.

I usualy just leave the display on Endurance cause I like knowing how far I'll go before needing gas. That is unrelated to average mielage. It's a rolling last 18 miles so reseting the average doesn't affect it.

[x97Melbourne]

I reset it when I fill up. It's just an estimation....and its close.....but as the other dude said, exact calculation is more accurate. But I'm just looking for an estimate....thats all i need.

[Tom K.]

At only about 1 1/2% high, my 128i avg. mpg readout is surprisingly accurate. Like others, I also reset it at every fill-up for a check on the miles to empty reading for each tank.

As there's no "right" way to handle it, you may also wish to reset it every few tanks to get a longer average, especially if you don't keep a record of your fuel cost & mileage.

Tom

[1Ronnie]

I usually do it by the tank, but I just did a 1400km road trip, so I reset it when I left, and didn't reset it till I got home. Use it how it makes sense to you.

[Tom K.]

Originally Posted by 1Ronnie

I usually do it by the tank, but I just did a 1400km road trip, so I reset it when I left, and didn't reset it till I got home. Use it how it makes sense to you.

If you iDrive, there is a trip function which allows you to do that independently of the OBC - unless you are like me and forget to reset it when leaving!
Tom

[gio]

Originally Posted by x97Melbourne

I reset it when I fill up. It's just an estimation....and its close.....but as the other dude said, exact calculation is more accurate. But I'm just looking for an estimate....thats all i need.

How can you do an exact calculation that is more accurate? There is no way for you know that you are filling your tank to the exact amount that you did last time so the car's average mpg display is always going to be more accurate.

[akak1997]

Originally Posted by gio

How can you do an exact calculation that is more accurate? There is no way for you know that you are filling your tank to the exact amount that you did last time so the car's average mpg display is always going to be more accurate.

if you reset your odometer after you refill every time, the number of "miles on odometer"/"gallon you just put in", will give you the exact mpg.

the computer is not counting how much gas you've used when you're sitting there idling. It won't be a lot, but it's still gas being burn.

[gio]

Originally Posted by akak1997

if you reset your odometer after you refill every time, the number of "miles on odometer"/"gallon you just put in", will give you the exact mpg.

the computer is not counting how much gas you've used when you're sitting there idling. It won't be a lot, but it's still gas being burn.

Yes, that is assuming you fill it up to the same level each time you fill it. This can vary as much as half a gallon since there is no way to be sure the tank is full. Maybe sometimes you have 13.8 gallons in your tank. Other times you might have 14.3 gallons. A half a gallon in a fourteen gallon tank will make your calculations off by at least 3.5% (and that's if you run the tank to empty each time).

It does calculate how much gas you are using when you are idling. Reset the gauge, drive a few blocks, then stop the car and let it idle and watch the average mpg go down.

[shmee150]

If you had gone say 300 miles, the amount you put back in until it's full is the exact equivalent to how much you used to travel those 300 miles...

This is the case if you always fill it.

Tim

[Guildenstern]

They are correct though that you can't always be positive that it has completly topped off due to slope of the ground, sloshing, or just a bad shutoff on the pump itself.

But I consistantly found that the Average MPG was consistantly *ahem* "Optimistic", (Just like the spedometer).

But I also like to look at trends and which gas isn't worth getting again, so calculating just gets done in the normal course of things.

[Tom K.]

Originally Posted by gio

How can you do an exact calculation that is more accurate? There is no way for you know that you are filling your tank to the exact amount that you did last time so the car's average mpg display is always going to be more accurate.

By recording fuel used, miles traveled and OBC MPG readings at each fillup. Over time it averages out so that you know both your true gas milage and also the % error for the computer.
Tom