Bigger Tires = New Tunes?

[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