Originally Posted by bandwagn
...+30 will give you about an inch more offset than the stock +56 (56-30=26mm=1")....
This is correct, however you need to keep in mind the rim width. Assuming you're using the same width rim, the offset is the only factor that will affect how far out the tire is pushed.
That said, I'll explain the rim width factor. If you have an OEM 18 x 8 rim with +56 offset, there is 1.79" from the face of the rim to the mount pad (6.21 inboard). If you buy an aftermarket rim that is 18 x 10 with a +30 offset there is 3.81" from the face to the mount pad (6.19 inboard).
The aftermarket rim may fit, because it won't hit the suspension, since the inboard distance is nearly unchanged. The issue though is the face of the rim (and outboard side of the tire) is pushed out 2" despite the offset of the new rim only looking like a difference of 1".
How did I get there, you ask? The offset is simply the distance off center that the mounting surface of the rim is located. So a +30 offset has the mounting surface of the rim 30mm outboard from center. If a rim is 10" wide, you'll have 5" inboard, and 5" outboard with a zero offset. To adjust for offset simply convert to inches (divide the mm by 25.4) and add/subtract that from 1/2 the rim width.
Make sense? If not I'll see if I can sketch something up.
Originally Posted by leadfoot4
I believe you have that information backwards. 265/60 is stock, and a 65 series tire will be the "taller" one......
Also true assuming the same rim diameter is used. For example the stock 20 is 260/55/20, but the spare is on a 18. The spare size is 245/65/18. Even though the spare is a 65 series, it's the same diameter as the stock 55 series tire, because the rim and width differences.
To go expand on that, since the OP asked about what the metric designation mean. I'll use the stock 265/60/18 for example. The first number is the width of the tire in mm. Just convert to inches, as noted above, and you'll know how wide the tire is. Stock would be 265mm wide or 10.43".
The second number is the "aspect ratio", expressed in a percentage. The 60 just means the side wall is 60% of the width. You can do this in metric or SAE. Since we've already converted the width, I'll use SAE. Take that 10.43 x .60 = 6.25". That's the height of the side wall. When using this number to calculate tire diameter, remember to double it, becuse there's sidewall on both side of the rim.
The last number is the rim diameter, in inches. Yes, they decide to through a SAE number in the mix for some stupid reason.
There you have it. P-metic markings = width / % / rim
As with the offset, if that doesn't make sense I'll try to draw something up for you.
Originally Posted by crombenevolant
For the tires, I am a huge fan of the DuraTrac's. I have them on my RAM and they are well behaved on road, awesome off road, and wear pretty good. In the snow, they are hands down the best tire I have ever had.....
I couldn't agree more. I also had them on a Ram, and loved them. I currently have Mickey Thompson MTZ on my Ram, and while they're nice, the Duratrac was better.
One thing crombenevolant didn't mention is that on the road they are also shockingly quiet for a capable off road tire. By far the best truck tire I've had.
FWIW, I've also had BFG, which were also great tires. I just like the Duratrac a little bit better.