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Discussion Starter #1
I just had to make a run down to the coast last weekend. I used the Predator 40 Tow tune with a tire adjustment for 265s. My sister was following me and we found out she was showing 2-3 mph faster than I was. I checked vs. mileage and she was the one that was correct. I was 3.9% low at 100 miles. My Predator had dropped the cal. I had heard about that before, but this is the first time it happened to me that I caught it. When I filled up to return, I had only gotten 17.1 mpg and that's adjusting for the offset. Let's just say I was extreamly disappointed as I have seen 19 several times and 22 once on a pure highway run. I returned to stock and drove home to yeild 18.9 mpg on the same trip back. I had quite a bit more stop n go through Houston and ran to deal with, yet still had much better mileage. I did try and load the factory tune with mileage adjustment again to see if it would stick. It appears to judging by RPM vs. MPH, but I'm not sure how to tell with such a small amount off on a daily basis.


Any suggestions on what to do? I was thinking about going with a Juice and using the Pred for tire cal, but if it won't hold, that's going to be really annoying. How would I tell without running 10 miles down the highway every week to check?


For now, I plan on staying with the factory power level and watching mileage for a couple of tanks. Then I will try the 65 or 85 and see how it does. The "custom" 120 I had, seemed to be doing well, but really looking at the data again, I'm no so sure. Most folks seem to show an increase or the same with the Juice, but in this case, the Predator was a definite drop off. That was not what I was looking for at all. I'd be okay with breaking even and having extra power, but 2 mpg loss is a bit steep.
 

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Check your speedometer against the highway mile markers occasionally. For 1 mile distance at:


55 mph == 65 seconds


60 mph == 60 seconds


65 mph ~= 55 seconds


70 mph ~= 51 seconds


75 mph ~= 48 seconds
 

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You can also use a GPS. If it reverts back to the stock tune, a 3 or 4 mph difference should be noticeable.


If you look at the manuals, the transmission sends the ECM a pulse train as a function of drive line rotation. The ECM then scales this pulse train by a factor that is stored in the ECM (thus adjusting for tire size). The ECM then outputs a standard pulse train that goes to the instrument panel and BCM (body control module). This is what I came away with from studying the manuals a ways back (so there could be some error here).


Not sure how this scaling factor could changed, I thought it was stored in the ECM. Maybe some of the various vehicle control modules are talking with each over an internal vehicle LAN? Some how there resetting each other? Since we do not know the LAN protocol/messages, I guess we'll never know for sure. Others have stated the tire size is also stored in a controller that works the ABS.


This has happened to more than one person, would be interesting to know why.
 

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Your speed/mileage differences pretty much match mine and I'm running 265's with no tire correction. My odometer is actually a bit "shorter" at 5% less miles, which is fine by me. Speedo is off 3-4 MPH at 75-80. I verified the speedo using GPS.


It's really not that tough to live with. Just drive 2-3 mph less than you really intend to (habits are hard to break!) and always add 4% to your odo for MPG calculations......
 

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chuntag95,


Every time I load a tune I have to reset the tire size for it to work. Change 315 to 320 and then back to 315.
 

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Discussion Starter #6
WanaDmxsub (me too),


Thanks for the tip. I will try it.


Due to the fuel mileage loss, I ordered an Attitude+Juice.
Cost me two birthdays, but I think it will be worth it.
 

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WanaDmxsub said:
Every time I load a tune I have to reset the tire size for it to work. Change 315 to 320 and then back to 315.

I have had the same issue after installing a new tune. My speedo would be off again. So, now everytime I install a new tune I then go to "modify performance" and "reset perameters" for the tire size and then re-enter the factory and actual tire sizes again to get my speedo correct. It takes a little longer than just changing the tunes but its the only way I can get it to work. I called Diablo Sport about this and they say that it shouldnt change when changing tunes ... "mine does". There is a new version out now that is suppose to have some new tunes in it and I may update to see if that will fix this problem. Basically, I've been happy with my Predator's performance.
 

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Rcher,


You shouldn't have to change everything. I just change one of the settings, like my example above, and it picks up the rest when it loads. I was told that it had something to do with the programmer not seeing the changes when you load a tune unless you actual make a change then it picks them all up. Not sure if v1.11 fixed this or not. I’m in the habit of changing tire size and always forget to check, and the batteries are dead in the GPS.
 
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