Battery Disconnect

So Monday of last week I put in new plugs and air filter thinking that MPG would change a bit and better then the 13MPG I was getting, I drove until last Friday and no changes at all. While packing up for a weekend trip last friday I remembered reading that disconnecting the battery would reset the computer and by doing so MPG would change. So while packing I disconnected the negative for 20 minutes or so, hooked it up again and off I went. I seen a change, HWY over 16 MPG and city now over 14 almost 15MPG depending on my foot. I'm not sure disconnecting is what made the change but if so it was worth the minute it took to disconnect and connect.

You dumped the computer putting it back in learn mode causing it to learn the new parts you installed..

are you using the display to determine the mpg ? I never do. First of all its not correct because of the larger tires, and then Id rather just do the math instead of relying on a computer display.

The computer display and your math would be corrupt because the odometer mileage is off do to the tires correct?

I use my display.

First off its not thrown off any more or less than the odometer is by tires.... and you're using the odometer to determine miles right?
Second, the engines computer is MUCH more accurate at determining the relationship between fuel consumed and miles driven. The resolution for fuel consumption in the engine is very refined..... however the "trigger" for gas stations are not. Just because you fill up and the gas pump turns off at 16.56 gallons does not mean that you have used 16.56 gallons. So dividing "miles driven" by amount to fill up is actually much more inaccurate. Some gas pumps are more sensitive than others so they'll stop sooner and therefore your "calculated" MPG will be higher than actual.

The computer however is very consistent and there gives the best tank-to-tank comparison of gas mileage changes.

That's how I look at it anyways. :)

You can easily adjust for new tire sizes by taking the MPG (calculated or computer) and multiply it by (new tire diameter / old tire diameter).

i dont use the odometer neither, i use gps.

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Originally Posted by NeilSmith

i dont use the odometer neither, i use gps.

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I used the GPS to get my spedo error so I can responsibly get some what of an accurate MPG. I'm running about 14.2 right now country hilly driving.

it just sounds funny to me that if the computer knew what it was doing then it would automatically detect that you changed the plugs and you wouldnt have to unhook the battery to reset the system. im old fashioned , i guess. :)

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Originally Posted by NeilSmith

i dont use the odometer neither, i use gps.

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GPS doesn't account for altitude changes. If you live in a flat area it's not bad.

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Originally Posted by NeilSmith

it just sounds funny to me that if the computer knew what it was doing then it would automatically detect that you changed the plugs and you wouldnt have to unhook the battery to reset the system. im old fashioned , i guess. :)

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The computer doesn't detect spark plug changes at all, not even unhooked. That just puts the computer back in to "learn" mode all over again and resets all of the sensors. If there are any changes that computer should make due to the plug change then it would eventually optimize itself, but it will take some time... you just force it to do it sooner by disconnecting the battery.

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Originally Posted by Matt_

GPS doesn't account for altitude changes. If you live in a flat area it's not bad.

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Depends on what brand and model you're using. I use a hand held for land navigation that does take hills and stuff into consideration as it tracks you mileage. I don't know to much about the ones mounted in cars though. I have to wedge it in the window sometimes I have to hold it in the window :( I hardly if ever use the nav unit installed in my jeep.

lol yeah i was just talking about the general automotive garmins and tom toms. I guess the higher end ones would help. He's still stuck with the inconsistency at the pump though.

I usually use my GPS to dial in my tire size so my computer is accurate. If i'm within 1mph of what my GPS says on a flat highway at 70 mph, i know i'm good to go. Then it doesn't get much more accurate than that.

If you get it at 1mph at 70 MPH thats pretty darn good. I don't think they come from the factory that accurate :)

They definitely don't!

One of the benefits of a programmer... tweaking the tire size in 1/4" increments comes in very handy. :cool:

I use the GPS for miles only as it does not calculate MPG. My GPS is very accurate, very close to the odometers in both vehicles I've had it installed in. I just think it is so easy to calculate mpg in your head when you fill up. The fact that the display needs time to catch up and correct itself after you change the plugs makes it harder for me to trust it. As far as the inconsistencies of the gas pumps, I understand that there may be a small bit, but I would think it is very minute. We all have to trust that the gas stations keep the pumps calibrated and this opens up another can of worms. When you pump 17.8 gal of gas are you really getting 17.8 gallons ? I simply like to calculate mpg the old fashioned way. I can do it in my head roughly and am usually pretty close. Or I just get my phone out which has a calculator. But whichever way works best for you. The worst part ? It can be kind of depressing when you see that number. LOL !

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Originally Posted by NeilSmith

The worst part ? It can be kind of depressing when you see that number. LOL !

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This is the exact reason my display stays on the temp/dirrection screen!! LOL...

I hear ya man.

But the MPG doesn't need time to "catch up". Thats not the point of disconnecting the battery...

And yes, when you fill up with 17.8 gallons you got 17.8 gallons. What i was talking about is the trigger that turns off the pump when you're full. That trigger is not as controlled... so one pump may put in 17.8 gallons but another pump will put in 17.2 gallons or 18.5 gallons. So if you went 270 miles "on that tank", depending on which pump you fill up at you could get a reading of 15.1, 15.7 or 14.6 MPG respectively. In other words, the amount that you put back in the tank isn't always the same exact amount you used.

In the end we're splitting hairs. It's not a matter of life or death... There's just that analytical PITA voice inside of me that's always nagging me. That voice goes a bit haywire in my head when i read someone post that their hand calcs are more accurate than the computer calcs, because when you start breaking down the variables involved, its really not. But it's also not going to be an order of magnitude off or anything... so yeah, im just going to go drink a beer and watch some hockey now :)

Oh, I gotcha. Yeah, its like splittin hairs. I dont stop pumping when the handle clicks off anyway. I fill it till I cant get anymore in there. Ha HA !! Im not saying either way is better or more accurate, I just tend to go back to simpler times when we didnt have computers to do everything for ya.

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Originally Posted by Matt_

GPS doesn't account for altitude changes. If you live in a flat area it's not bad.



The computer doesn't detect spark plug changes at all, not even unhooked. That just puts the computer back in to "learn" mode all over again and resets all of the sensors. If there are any changes that computer should make due to the plug change then it would eventually optimize itself, but it will take some time... you just force it to do it sooner by disconnecting the battery.

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Thats is the reason I disconnected the bty, after 4 days of driving and filling up twice due to the amount of driving I did, I saw no change at all ( Computer or how many K's I was getting per tank) I don't completely trust the computer with how much MPG i'm getting but when the number didn't change I though I'd try it and it seems to work, same drive after disconnecting I got almost 100K's more. Now that fishing is open I will be going to the cabin every weekend and I'll see how much k's I am still getting to a full tank.
While looking at the MPG is not a modivational poster it doesn't matter when I pass 3 stuck Toyotas stuck, yes 3 and just drive around them to get to my fishing holes, it worth every $/gallon

Just some info from my experience, after a few weeks of driving, I am getting the same mileage and even a little less then before I disconnected the Bty, so this urban myth of getting better mileage by disconnecting the bty was false. Maybe Mythbusters would have better luck then me.