Tire Size not Changing

[ak_cowboy]

I've upgraded my tires to 295/70R17s (33.4") and my speedo is reading 9% slow. This is GPS vs what the Gryphon says. I had the same problem when it was the stock edge, I can adjust all the other custom settings, but the speedo is always off no matter what numbers I put it. I've tried from 2500 to 2800 and its always reading slow. ex says I'm doing 55mph while gps shows 60mph...

Any idea how to fix this?

[Jackpine]

All right, this sounds like you're not going into the load tune state after you've entered the Tire Size (TS), or the your Gear Ratio (GR) value is wrong. BOTH will have "default" (placeholder) values when you first open the custom options menu that may or may not be correct.

I can't hep you with the GR value, you have to get that off the VIN sticker. But I suggest a TS value of 2573mm. Your unloaded tire circumference is about 2653 and reducing it by 3% for the flattening effect of weight gives 2573.

Now, when you open the Custom Options Menu: scroll to GR and put in the correct figure if it's not already there. Hit [Enter] to "save" it. Scroll to TS and put in 2573. Hit [Enter] to save it. If you push the [Menu] button in either case, you will "back out" without saving. Then, scroll to "Done" and hit [Enter] to load your tune and those values into your PCM.

Test your speed after you get the tune loaded. If the Gryphon still says you're going slower than you really are, increase your TS. But, this time (and any future time) you open the Custom Options Menu, scroll to "Load Previous Settings as the very first step you perform!. Hit [Enter] to retrieve your previous TS and GR (and any other changes you made. Now, when you scroll to TS, you should see a value very close to what you last used - it won't be quite the same due to "rounding", but the difference will be minor. You can change it as needed.

If you don't see the proper TS after loading previous settings, or if you skip the "Load Previous Settings", you're doing things wrong. Re-read these instructions.

- Jack

[ak_cowboy]

ok, I'll give that a shot and see what happens.

Thanks

[Longshot270]

The easiest way I have found to get the tire size accurate with a GPS is to set your tire size to something you know is wrong, like 2400, clear the odometer on your truck and on the GPS, drive 10 miles then input the numbers as shown.

(GPS Miles/Truck miles) X 2400 = New Tire Size

[ak_cowboy]

So I started at 2573 with my gears of 4.10 and was showing 51mph while travelling 55. Shut the truck down, loaded previous settings, and changed the TS to 2774, speedo didn't change. Loaded previous settings again, upped TS to 2900 and speedometer was still off by 9%....

[Jackpine]

Originally Posted by ak_cowboy

So I started at 2573 with my gears of 4.10 and was showing 51mph while travelling 55. Shut the truck down, loaded previous settings, and changed the TS to 2774, speedo didn't change. Loaded previous settings again, upped TS to 2900 and speedometer was still off by 9%....

OK, do it again. This time, tell me what your Tire Size value is right after you "Load Previous Settings" (before you enter anything new). Also, if you DO have 4.10 gears installed, tell me what the Gryphon shows for Gear Ratio after the "Load Previous Settings" step.

If you're doing things right, you should see a TS value that is "close" to 2900 and a GR that is 4.10 at this point. Tell me what you find and we'll go from there.

- Jack