Sorry for the typo, they are 275/70/18.
Long weekend, my fingers won't cooperate this morning.

FREE SOFTWARE...
 

Just in case, here's an Excel spreadsheet that eliminates all that math.

Well, not exactly "eliminate", but at least automates...

All-righty, if your speed was reading slow, the Tire Size (TS) you were using before was a bit too small. An error of 2mph in the 70mph range is in the 2-3% range. IF you were using 2560mm before, you'd increase that to 2611 - 2637mm.

My guess is though, you were not using 2560 earlier, but had entered something smaller? AND, when you enter the TS, also make sure the Gear Ratio (GR) is correct. The older Gryphon uses "placeholder" values for both TS and GR in the Custom Options menu, and BOTH of them can be wrong.

- Jack

I would ball park it in the mid to upper 2500s by using the gear ratio calculator in pegasus. You just type in the tire size and double the amount of squish. Anything that is more than 50-100 from there is probably incorrect.

I'm getting a zip file error on this download.

When I make the change...should it be a permanent part of that tune? When I changed it before, I used the 87 tow tune and changed the tire size to 2646. Then I changed it back to the #1 tune...and all was OK, If I go back to #2 will the 2646 tire size be there again?

Originally I didn't change anything...I said "NO" to custom settings, that is when the 2MPH difference occured...in the #2 Tow setting. I am not sure what the default tire size is for the #2 tow tune, but that is where I started....then changed it to 2646 and it went way out of wack.

Try programming to the tow tune without making any custom changes. Then the next time you program, after you select "yes" for the custom options menu, go directly to "load previous settings" and see what value is in the tire size. I dont think it puts a placeholder value when you do that...at least it doesn't seem like it on mine.

Sorry, that won't work. He's got a "previous settings" file now and when he reloads it, he's going to get the 2646mm value.

So, by bypassing the Custom Options choice, you actually got a pretty accurate value, the one that's held in the PCM's VID area. However, there's no way for you to find out what that is! Seems weird, but that's just the way things are.

Here's what I'd do then. The Gear Ratio Calculator in Pegasus gives the 2560mm value assuming a 7% rubber squish (deformation due to weight on the tire). That's about the same as a 3% overall reduction in the unloaded circumference of 2646mm. I think 2560mm is a good starting point and is the value I'd put in for TS when you reload the Level 2 tune. Make sure the Gear Ratio is correct too. Now, when you let the Gryphon program your PCM with those values, they will be the ones saved in your "previous settings file".

Whenever you change your tune in the future, you can select Custom Options and then "Reload Previous Settings" as the FIRST step! This will replace the placeholder values with your saved TS and GR values.

If you discover the speed is still slow, increase the TS a bit. If the Gryphon is registering fast, you'll need to reduce the TS a bit. To give you an idea how much, a 1mph error at 70mph would call for about a 37mm change.

- Jack

How did you guys get so smart? I'm sure glad I found this forum.

I guess my next step is to figure out what my rear gear ratio is.