wheel fitment 101. E-mail me for advanced courses such as, "Difference between LIP and DISH (Because there IS one!)" and "I love deep LIP wheels, what factors give me a deep LIP"...
Terms you need to understand:Pad - The material on the back of the wheel that bolts up to your car's hub.
Offset - Measured in Millimeters, this is the distance from the center of the wheel to the PAD.
Backspacing - Measured in Inches, this is the distance from the back lip of the wheel to the PAD.
Stretch - This is when a narrower tire is forced on to a wider wheel and the sidewalls are literally stretched to bead. *Most tire places WILL NOT mount stretch tires due to some reasonable concerns, mainly the tire unbeading and violent blow outs. I have seen stretched tires both unbead and get punctured. My brother drove like this cautiously for 2 miles to get off a bridge. Him and the car were fine. Just saying, I have never seen these concerns play out (thankfuly)
Now that these basic terms are laid out, the term fitment is more a matter of opinion than fact. Generally though, it is referenced at the current trend of making a tire as flush to the fenders as possible. In doing this, many sacrifice drivability and turning radius due to clearance issues. The most extreme cases also sacrifice tire performance due to STRETCHING their tires to fit the most aggressive wheels possible. Others try to optimize tire performance by fitting the most rubber under the car. This is still technically fitment, just not the trendy style right now. The trick is to find the equilibrium for your self between form and function.
Offset and width relationship:Because the way OFFSET is measured, width of the wheel does change the over all fitment, even if offset is unchanged, and the opposite is true that changing offset but not width will also affect over all fitment. It all comes down to simple math. If you have a 7" wheel, and want to go to an 8" wheel, you are effectively adding .5" of wheel on both sides. That extra material is what is taking up the extra well space and creating better fitment. *If you have coilovers, you have to be careful because those take up more room than normal shocks, so a wide wheel (9" or so) with a high offset (+30ish) will not clear the suspension.
Here is one offset calculator.
http://marksink.com/tire_wheel_offset/offset.html
Also, this website is a good general learning tool, and has a chart converting backspacing to offset.
https://www.rsracing.com/tech-wheel.html
Thanks for reading my novel, again, feel free to e-mail me any questions. Or if anyone has general questions that would benefit the discussion, post them and I will be more than happy to reply.
Modified by TIMELSS_vert at 12:24 PM 5/24/2009