Bigger Tires = New Tunes?
 

I just put some 295/75/18 tires on my truck and wasn't sure if needed to get new tunes?
I feel the shift points might need to be adjusted

No you can adjust the tire size through the custom options menu when ever you reprogram. Try entering 2750 as your tire size and see how close that gets you.

Or the best thing to do is to program a value you know is incorrect like 2600. Then if you have a GPS going clear both it and your odometer and drive at least 15 miles. Then do this little formula:

GPS odometer/truck odometer x the tire size you just entered (2600). The number you get is your actual tire size.

Use the custom options menu to change also double check the gear ratio...

size
 

so would the same apply if it was a 295/65/18? Its still an 18? Thats cause I just out my new tires on too. So under greate custom option hit yes? and just fix the tire size to ??? and hit yes thats it.?

No, a 65 series tire has a much different diameter and circumference than a 75 series tire. Longshot gave you 2750 for the 75's, I would have suggested 2730, but that's a small difference that might be good "boundaries" on the values to try. If you've installed a 75 series tire, start with a value between 2750 and 2730.

If you are using 65 series tires, they are not nearly as tall (but they're "fatter"). Try something between 2580 and 2560mm.

You would enter these values in the Custom Options Menu - Tire Size.

- Jack

Ok. I'm going to try. But why the range ? Is it to get as close to actually displayed speed?

I'm giving you a "range" as boundaries on the value I think you should use. Different tires will exhibit a different amount of "squish" under loading due to internal construction. And the inflation pressure you use will also effect the amount of squish you experience, as does the aspect ratio of the tire.

All in all, you're going to be VERY close with the middle value on either of those ranges, depending on the tire you mount. If you want to really "nail" it, you're going to have to adjust to make the Gryphon displayed speed match a GPS stabilized speed (near 70 mph is best), or, get your odometer to match a known distance driven, the longer the better (like about 100.0 miles).

The dashboard speedo will generally read about 2-3 mph faster than your true speed when everything is set right, but the odometer will be correct.

- Jack

I also recently added larger tires(today)...265/70R-17 Nitto Terra Grapplers along with new pro comp wheels. I reset the tire size to 2521 after following the recommended formula per my instruction manual. There was a vague reference to possible gear ratio changes due to larger tire size in my manual. Should I be adjusting this value?

No Kirt -

You make sure the Gryphon is using the actual Gear Ratio (GR) that is INSTALLED in your truck.

I'm not sure what "reference" you are talking about though. It COULD be that installing larger tires reduces the "effective Gear Ratio" on your truck, which it does. Or, that to regain the "stock GR feel, power, etc." after installing larger tires you would have to install higher gears, which is also true.

- Jack

Thanks Jack....I do think it is talking about changing the actual gears....like I said it was kinda vague