Hi,

I have a 2006 5.4L F150 XLT 4X4 with custom VMP tunes. It has 2" Bilsteins and and add a leaf in the rear. I will be putting on larger tires in a month or so...stockers are 32" and I will be moving up to close to 34".

I know that I just change my revolutions per mile in my XCal 3 and reflash the truck when I put the new tires on.

My question is what happens when I take it to the dealer? I always reflash my truck to stock before taking it in for service. If I do that my speedo will be off by quite a bit and it will not be shifting as well. Is there a way to only modify the revs per mile on the stock tune when I set the truck back to stock or am I stuck with it being off? I guess it shouldn't be a big deal for only that short time frame but I figured I would ask since many others here must deal with this issue some how. Any advice is appreciated. Thanks!

 

It will be off by about 6% for a day. Not too painful.

 

you will be fine for the short trip to the dealer. or put it back to stock at the dealers and take your SCT with you. and when you pick it up tune it again. not a big deal

 

I am doubtful there could be a way to only modify the revs per mile on the stock tune when you set the truck back to stock

 

I figured I would be fine since it would only be a few miles off at 60 mph. I just thought others on here might have had a way to deal with this problem. I see no reason to chalk up the money since I have the X3 but can't the dealer change the stock tune for larger tires? If someone does not have a programmer and puts on larger tires I would think the dealer could do it but they charge a lot of money for something so simple. I think I'll just deal with driving around for a day with it off about 6%. Thanks everyone for the responses!

 

Brings up a question...do the shift points really change? Are the shift points dictated by the speedo or RPM's? I have a TP tune and I'm pretty sure it is rev's not speed.

 

Good question. I am curious myself. Speed and RPMs are connected though...if you put a larger tire on then you will run the engine at lower RPMs for a given speed. If shifts are based on RPMs then correcting the speedo will do nothing for my shift points. If they are based on speed then correcting the speedo will help the shifting with larger tires. Can anyone else chime in on this?

 

Normal, part throttle shift is controlled by a kind of "sliding" valve that is effected on one hand by speed, and on the other by throttle position. As the speed increases, with a constant throttle position, the slider moves toward the point where a shift up would normally occur. But if you increase the throttle, it pushes the slider the other way, delaying the shift, or, if the upshift has already occurred, it causes a downshift.

I'm kind of simplifying here, but this is the basic control system. It's a balancing act between speed and throttle. It is not governed by RPM unless you are in a WOT condition (somewhat above 90% open throttle). Then, it's totally controlled by RPM.

- Jack

 

When the truck is tuned to stock, the Tire Size (TS) and Gear Ratio (GR) are taken from the PCM's Vehicle ID (VID) section and these are the OEM values it had when it left the factory. A custom tune can override these values and you can do it manually if you've set the PCM to a non-stock tune. If a dealer reflashes your PCM to correct for non-OEM tires or gears, I suspect it COULD change your PCM's HEX code. If this happens, your custom tunes may no longer work since they are built to match the "stock" program in your PCM.

I'm guessing that a dealer reset of TS or GR might change the HEX code, it may not. I DO know if a PCM is reflashed it causes the custom tunes in a Gryphon to be rebuilt. I think it's reasonable to assume the same might be true of an X3 or other custom tuned programmer if the HEX code changes.

- Jack

 

p185, the info used for tire size and gear ratio is stored in a seperate data table than the timing and fuel uses. When you flash the trucks PCM, you have the option to alter the TS and/or GR. When you change them, the new values stay even when flashed back to stock. So you could reload any of your tunes, adjust your TS/GR, and when you go to the dealer, put your truck back to stock and your TS/GR will not revert back to stock. The only time these values changwe is when you change them.

Bill (ASE Mechanic) knows exactly what I'm talking about and could probably explain it better than I can.

 

Thanks for the explanation Red. That means I'll be good to go after I change the TS with my X3. Based on Jack's response I also see that my shifts will be affected when I correct the speedo since part of the shift depends on speed and part depends on RPMs. Thanks for all the help guys!

-Pat

 

Red, I don't want to get into any kind of "I know better than you" match here, but you've just said something that doesn't seem to square with what I've learned from Bill. And, if I learned it wrong, I'll be most happy to admit it and will try to undo some of the misinformation I may have been spreading around.

Here's what I think Bill has told me:So, I'm just not sure what you said in the part of your quote that I bolded is correct. And, maybe different programmers handle this differently too!

As I said, I'm not trying to start a fight, but would like to find the truth in all this!

- Jack

 

Hey Jack, no offense taken. I was trying to explain it in laymens terms to the best of my ability. I'm not sure exactly how the whole system works, thats why I'm not a tuner. But I do know that the TS/GR info is stored in the VID (as you called it) and can be changed along with the tune or by themselves. Once the TS/GR is changed, the new value stays until you change it again, regardless of what you do with the tune, such as return to stock, or load another tune, or leave it alone.

I guess the best way to look at it is its like the PCM has 2 EEPROMS, one for the tune and one for the VID, each can be reflashed independently or together. It's almost like flashing the BIOS on your PC except the PCM has a dual bios that stores different data in each versus a PC w/dual bios that stores indentical data in each (mainly as a back up in case of corruption)

 

Ah, that's where we see things differently! Bill told me (once upon a time, and I can't remember when or where), that the VID CANNOT be changed by a tune. So, it's going to give each new tune OEM values for TS and GR. This makes sense if you think about it. The VID contains vehicle specific information such as the VIN, and you would not want that to be overwritten. To emphasize, the TS and GR in the VID are NEVER changed, but they can be ignored or bypassed.

If, of course, you can flash the VID section, then anything I'm saying is completely false. I'm certain Bill told me that is not the case.

We'll get to the truth of this matter eventually, won't we?

- Jack

 

Like I said, I'm not exactly sure how it all works, but if the dealer can change info in the VID block (like TS or GR or other vehicle specific info) then my best guess is you can reflash the vid. Whether the egde (or X2, X3)actually reflases the vid block or tricks the pcm into thinking it has really doesn't matter. As long as we know that the altered TS/GR data will stay when you go back to stock.

Kinda like 6 of one half a dozen of the other, no matter how its done well get the same result in the end.