Don't Forget to Figure Tire Size When Calculating MPG!
#11
09-14-2009, 08:16 PM
On a truck with ABS it would be easy for the PCM to recalibrate itself. It is measuring wheel rotation and speed all the time and it can compare distance traveled to see if there's any change and readjust. Like I said, I don't know if it can do this, but there are a number of sites where people have said it can. I know the history of my truck since it was new and the original owner says he never had it recalibrated.
Last edited by jasonw; 09-14-2009 at 08:19 PM.
#12
09-14-2009, 08:28 PM
It's certainly possible that the PCM can recognize a change in tire size since it monitors tire rotation, speed, rpm and mileage constantly. Whether it can correct for changes is the question. I have read that up to a certain size, it can, and that over that size it can be recalibrated easily by entering a new number in the PCM via the Scan tool at the dealer. This number is supposedly configured using the distance in feet that the tire travels in one rotation. It would be easy to have a range of these numbers stored in the PCM for minor adjustments, but need a recalibration for tire sizes outside the range. Of course, this all assumes that Dodge engineers were this bright back then.
#13
09-14-2009, 11:55 PM
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It's certainly possible that the PCM can recognize a change in tire size since it monitors tire rotation, speed, rpm and mileage constantly. Whether it can correct for changes is the question. I have read that up to a certain size, it can, and that over that size it can be recalibrated easily by entering a new number in the PCM via the Scan tool at the dealer. This number is supposedly configured using the distance in feet that the tire travels in one rotation. It would be easy to have a range of these numbers stored in the PCM for minor adjustments, but need a recalibration for tire sizes outside the range. Of course, this all assumes that Dodge engineers were this bright back then.
I actually believe what Miami is saying, because a month ago I finally got my 33's and was very concerned about recalibrating the speedo and such since I had 225/75/16's on before the upgrade...well last week I went past one of those "Your speed is" zones and guess what? It was spot on exactly what I was reading, and I went back a few times to check at different speeds and it checked out EVERY time....imagine that! So now Im no longer in the market to have it recalibrated :-)
#14
09-15-2009, 01:17 AM
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My tire size is 245/75r16 which equals a 30" tire. After reprogramming to a 30" tire with my Superchips programmer my speedo is dead on. As I said before, before that I was doing around 58 when my speedo said 55. I don't believe that a pcm can automatically recalibrate. One of those radar things they put up may not be calibrated correctly. If you put bigger tires on and your mileage was reading correctly then the speedo was off to begin with. And people wonder why I say not to believe factory guages.
