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

[sblue]

So like my title says I made a switch from my stock 275/65R18 to a new wheel/tire size of 305/55R20.

After doing a bit of driving, it appears that nothing has changed from my speedo to my Gryphons speed? My speedo has always read 1mph faster than my Gryphon up until about 65mph then my speed is about 2mph faster than the gryphon.
I got the same exact readings after my tire change. I though my speedometer would be slower with a tire roughly 1 inch taller?

Am I supposed to adjust the gryphon first and then compare?

What about tire squish, does that change with a bigger wheel? (20")


Is there anyone here that has made this switch and re adjusted your gryphon?


I'm horrible with numbers and I'm hoping someone here can give me a correct number to input into my gryphon.

From the tire calculators I tries I came up with the stock size at 32.07" and the new size at 33.20". This i believe is unloaded without squish.

I have a feeling even though my gryphon and speedo haven't change that I still need to make some kind of correction.

Many many thanks guys!

Jack&Janet? You out there? I know your the guru's around here and could probably just wave a wand and give me the right size to input. Lol

Again, thanks guys!

[sblue]

Bump, anyone?


I have another question. Does my Gryphon already have the tire height value stored in it?
I ask because I need to know if that number needs to be higher or lower with my larger tires. So if stock is 2700( just an example, not sure what stock is?) would I go higher or lower?
Also does my squish factor change with 20"s?

I've done tons of searching but the majority of threads are about changing to 35" tires. And also, I'm horrible with numbers and other than the calculator I used in my above post to get me the tire height difference I'm totally clueless on the system one would use to determin what new number I need to input along with the squish factor????

Sorry if I'm beating a dead horse here guys. There's just not anything I could find online with my old to new tire combo that has made the correct adjustment?


Again, many thanks for anyone's help.

[Longshot270]

If you are going by height I'm assuming you have a CS/CTS?

I'm gonna guess somewhere around 2600mm circumference or 83.3 cm in diameter (32.8 inches). If you have a GPS, clear both odometers and drive at least 10 miles. Then enter the values as shown.

Truck Odometer / GPS odometer x (the tire size you've had entered)= new tire size

[sblue]

I have the original edge/gryphon.
I also don't have a gps, I do however have a gps app on my iPhone. Will that suffice?

I've never entered any tire size figures in my gryphon, does it already have a stock number for me to work with in it?
If so how do I find it?

So what I would do is reset my trucks trip and start a new trip on my iPhones gps travel for at least 10 miles?

Once I have my figures what do I do then? I'm sure they will both be different.

[sblue]

I have the original edge/gryphon.
I also don't have a gps, I do however have a gps app on my iPhone. Will that suffice?

So what I would do is reset my trucks trip and start a new trip on my iPhones gps travel for at least 10 miles?

Once I have my figures what do I do then? I'm sure they will both be different.

Another thing is when I have the right figure and am ready to input it into my gryphon will I delete my custom 87 performance tune?
I have to make the change in that setting right?

[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