275/65R18 To New Size 305/55R20, Please Help

[sblue]

I wish there was an easier calculator that would give you how many mm you should adjust for per inch of tire you change.

Is driving 10 miles the only way to do this?


Sorry guys, I'm sounding like a total knob. As you can tell in clueless.
I cant even find out how to see what the stock tire size is on my gryphon?

Totally lost and just might throw in the towel and let it be.

[Longshot270]

Yeah, the Iphone will work. You can also go to google maps and get directions from one place to another. If you did not enter your original tire size then program 2400 for the tire size, drive, then do your calculations. Doing this will get your tire size within a few mm.

Another way is to mark the ground and your tire, roll forward 3 rotations, take the length (in meters) and divide by 3. That will give you one average circumference. Then multiply by 1000 to give you mm.

You can't really add a number of mm of circumference for every inch of diameter because different tires have different attributes. Also, tire pressure will have a large influence.

[sblue]

Thank you for all the help long shot.

Going back to my first post. If nothing has really changed between my trucks speedo and gryphon should I even bother?

[Longshot270]

They both use the same value for their speed calculation. If one is off they are both off. The needle on the truck is designed to be about 2 mph off at 70.

[Jackpine]

Since you have the original Gryphon, you should have downloaded Pegasus. Pegasus has a "Gear Ratio Calculator" that can be used to determine your Tire Size (including squish). Click on "Calculators->Gear Ratio Calculator" to start this application. Now, enter your tire information this way: "305/55-20" (notice the "dash" before the 20) in the "Old Tire Size" box. The application is set up to apply a "rubber squish" value of 5% (you'll see that in the "squish" box). Once you've entered your tire's numbers, you'll see a value of 2596mm in the "Old Circumference box. Use that as a starting value - I think it's going to be nearly perfect and you'll note it is almost the same as the 2600mm figure Longshot gave you. (4 mm is less than 3/16 inch, by the way.)

Your 55-series tires don't really "squish as much as the more narrow aspect ratio ones. For those, I'd change the value in the Squish box to 7%.

As Longshot said, the truck's speedo and the Gryphon will always be different by the same amount. They're both getting the same information from the PCM, and the truck's speedo just has a "high side error".

You won't change your custom tune by correcting the Tire Size. Just open the "Custom Options menu" when given the chance and then scroll to Tire Size and enter it. Hit <Enter> to save it. Then, scroll to Gear Ratio and change that value to whatever your axle ratio is (if needed). You may not have to change anything there, but you need to check it. Again hit <Enter> when it's correct. Then scroll to "Done" and hit <Enter> to start programming.

- Jack

[sblue]

I went on php and printed out there gryphon manual/instructions.
On pg. 15 is a calculator on how to do this. So I plugged in my stock tire info and got 2483 (this is with the pho squish factor of .97) I also plugged in my new size (305/55/20) and got a reading of 2570.44

So if my 20"s give me less squish factor should I multiply by .98 instead, would that be about 7% ?
This would give me a reading of 2596.94 (2597)

So one last time to sum it up guys, I go to my 87 performance tune, hit custom options, scroll to tire size and enter either 2570 or 2597 (depending on which is more accurate for squish) hit enter then scroll to gear ration and ensure it says 3:73 and then I'm done?

I realize this isn't done with a gps, but this will get me closer to accurate than what I have now right?

[sblue]

Just to make things easier i think I'm going to to with the value I calculated on the gryphon instructions of 2570

I believe that's with a 5% squish factor

I'm not sure how to use a 7% squish factor with my 20"s? Would 2596 be more accurate?

[sblue]

Great! So I went to make the adjustment and I got an error code (f1)

Vehicle calibration is not supported?

And to top it off it's stuck on this and won't return to the mani menu or even shut off?!?


Help!

[Jackpine]

Don't panic, the error code 0xF1 can sometimes appear during programming. Try this:

1. Leave the ignition key in the ON position.
2. Disconnect the Gryphon at the OBDII port.
3. Plug the Gryphon back into the OBDII port.
4. After getting to the main screen, wait 15 seconds for needed parameters to be loaded.

If you still cannot program the PCM successfully, contact PHP for assistance.

If things seem to have recovered, try reloading your custom tune and use the Custom Options menu as I described. I would use the 2596 value though. The 55-series tires do not "squish" as much as others.

One more thing. BEFORE you try programming again, make sure your battery is fully charged. With the problems you've been having, the battery could be down quite a bit. (When the Gryphon does not shut off, it pulls a rather large amount of current.)

- Jack

[sblue]

Ok I was able to get into programming and insert my new value. I followed the instructions and when it was complete I went back in to check the changes. Well it still read the original value which is 2356.

It went through all the reloading procedures so why is my new value not in there?

Help! Lol