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Discussion Starter · #1 ·
I bought a 19 bighorn midnight edition and absolutely love it . It has the regular suspension On it and is trying to see what the biggest tire I can fit without rubbing . I need some snow tires for the winter and wanna get 33/12.59/20 , but look huge and I wanna make sure they don’t rub.. can someone give some insight and possibly pictures of similar trucks at stock height ???
 

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I dont know much about the new generation - I'm assuming fiat didnt change the front suspension and wheel wells too much.

I run 315x70/r17s, the same as the ford raptor, which is approximately 34x12.4 inches. Without a level, they just *barely* clear:

trook11.jpg

steering at full lock:

trook12.jpg

this is after a 3 inch level:
trook10.jpg


Its important to note that these trucks are built with VERY small allowances: any deviation from its factory specifications will affect how the truck performs, particularly when its loaded.

  • Your speedometer will be approximately 4-5 mph slow with the new tires. Anything over 5 mph will affect how the transmission shifts. The dealership has the capability to recalibrate the speedometer but will more than likely refuse to do it. Rough Country makes a little programmer that corrects it.
  • The drivetrain does not care about the additional stresses imposed by the larger tires (this is supposed to be a truck, which is supposed to be engineered to handle these stresses)
  • Larger tires will reduce the RPMs the engine spins in a given gear at a given speed (simply put). Depending on your gearing, and because you have the 8 speed transmission, this may result in the transmission "hunting" for gears under loads or at highway speeds.

    Lemme try to explain:
    Without starting a giant discussion about *optimum gearing*, just know that an internal combustion engine is the most efficient at about an 80% load. Therefore, the transmission attempts to keep the engine at an RPM where it would be subjected to this optimum load at any given speed. When you need more power, it downshifts to bring the engine closer to its power band (where it makes maximum torque and HP: in 2013 this was between 4k and close to redline), then once you let off the gas, it'll upshift to a higher gear and return to normal.

    Increasing the number of gears (from 4 speed, to 6 speed, to 8 speed, and so on), allows additional "options" for the transmission to "choose" from when driving (because remember, the engine is most efficient at an 80% load - so for example, It'll be more efficient going up a certain hill at 2000 rpm instead of 3000 rpm).

    This decreases the "spacing" between the gears, which consequently decreases the "allowances" to things like larger tires. For example, at 77 mph in 4th gear with stock tires, my truck's engine spins at 3000 rpm. With the larger tires, the engine spins at 2750 rpm at the same speed and gear. The engine may not be running at its most *efficient* speed, but the transmission will maintain a constant gear regardless of an increase of load (like a hill, gust of wind, etc)

    Lets suppose at 77mph, the truck was at 1800 rpm. The larger tires now make it run at 1550 rpm. Now the engine is under a higher load, and might not be able to maintain speed when a minor hill or a good gust of wind hits - so the transmission will be playing this annoying ass game of "hunting gears", trying to find the "perfect gear" that no longer exists, because the tires upset this tightly engineered balance.
 
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