I believe the stock rims are 18 x 7.5 +44.45, which puts them at a 5.5 backspace. Using the stock 255 wide tire on the stock rim, the inside edge of the tire will be roughly 10.5" inboard of the mounting face. If Dynatrac is using a 4.75 backspace with a 315 wide tire, their inside edge will be roughly 11" inboard of the mounting face. That's a lot of confusing numbers to some people, so what does that mean to you scoobycarolanNC? Simply that Dynatrac's setup puts the tire approximately .5" closer to the suspension than the stock rim/tire combo. Therefor using the stock rim, you can mount a 275 wide tire and have the same inboard clearance as Dynatrac's 315 on aftermarket rims. The nearest size that comes to mind is 285/70/18 which is actually 34" rather than 35" if that matters to you. However this size is still bigger than stock (1.7") and available in the BFG T/A KO2, which is a great tire.
If you want to calculate any of this the math isn't that hard.
- First calculate the backspace of the rim. This number represents the distance to the inside edge of the rim, in relation to the mounting face against the suspension. To do this take half your rim width (7.5 / 2 = 3.75), and add the offset (44.45mm = 1.75"). Total backspace is then 5.5 for this rim. (3.75 + 1.75 = 5.5")
- Now you can take the tire width and convert to inches. The first number on your tire is the width in mm. Stock is 255. To convert to inches from mm divide by 25.4. (255 / 25.4 = 10.04")
- With the tire width in inches you can divide it in half (because on the rim half goes inboard, half goes outboard). (10.04 / 2 = 5.02")
- Lastly take the 1/2 tire width from the last step, plus the backspace from the first step and you get where the inboard edge of the tire is in relation to the mounting face. (5.02 + 5.5 = 10.52")
If you want to see how different width tires will compare on the same rim replace the orange numbers with those from the new tire. So for example a 315 tire would be 6.2" (follow steps 2 and 3). Replace the orange 5.05 with the new tire's size of 6.2. The new tire will be 11.7" inboard from the mounting surface, or 1.18" closer to the suspension than the stock tire (11.7 - 10.52 = 1.18").
The same thing can be done to experiment with different rims by adjusting the green numbers, which will change the red number.
Okay, math class over. I hope that helps though.