Bigger Tires = New Tunes?

[Jackpine]

Originally Posted by bikerfuzz

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

[bikerfuzz]

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

[Jackpine]

Originally Posted by bikerfuzz

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

[KIRT]

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?

[Jackpine]

Originally Posted by KIRT

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

[KIRT]

Originally Posted by JackandJanet

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

[Jackpine]

Originally Posted by KIRT

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

Great! If you've ever had to write a "user manual", you'd soon know how difficult it is to mean what you say to everyone. Larger tires effectively reduce your Gear Ratio. Smaller tires do the opposite.

The Tire Size (TS) and Gear Ratio (GR) settings in the Gryphons are there to make the speedo and odometer read correctly, and to make the transmission shift at the proper points in normal, part throttle shifts. The settings DO NOT restore any "loss of power" you get by upping the TS (reducing the GR). The custom tunes CAN help, but only to a point. I've got other posts in the forum that talk about the effect of increasing tire diameter.

Essentially, a 1 inch increase in tire diameter in the 32-33 inch range is going to cost you about 4% in perceived power. A 2 inch increase costs about 9%. You can really only get that back by changing axle gearing.

- Jack