When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Let's say that you choose a 12" wide tire. Okay, now let's take three wheels (7.5", 8.5", and 9" wide) with IDENTICAL 5.5" backspacing. The sectional width of all three tires when mounted should be roughly the same. Correct? And all three should fill the same space when mounted on the truck, with none sticking inward or outward farther than the other regardless of rim width. Correct? The rims themselves would stick out farther, but since they are less wide than the actual tire, the overall width should stay the same by my reckoning.
Not quite. Back spacing is a measurement from the hub to the back of the rim.
Based on the offsets you've provided, if you plug them into an offset calculator, you'll see the 9" wide rim is identical position for the inner lip, but the outer lip sticks out 38mm more. So even with the same 12" wide tire on both rims, the wide rim will stick out more.
Offset is a measurement from hub to center of rim. Your theory would be more accurate if we were discussing offset, instead of back spacing.
Last edited by ExpresswayFord; 02-05-2017 at 02:24 PM.
But, the overall width would stay the same since the tire is the widest component, wouldn't it? I definitely need the inside lip to be in the same place to prevent UCA rubbing, but it appears to me that wheel widths less than a tire's width should result in tires sticking out the same distance from under the truck if the backspacing is the same.
With backspacing constant, offset decreases as the wheel's width increases. Correct?
Correct. With your backspacing constant, the center of a wider rim has to be closer to the hub, meaning a smaller offset measurement, which translates to the outside lip having to stick out more.
But yah, a 12" wide tire is a 12" wide tire regardless of what size rim it's on. It will appear to "balloon out" more on a narrow rim, however.
I bought 295/70/18 Cooper S/T Maxx tires for my leveled '05, but I do not want to use the slightly-corroded stock wheels, so I'm trying to decide between 8.5" and 9" wheels and get the offset right to where I don't rub. To match my stock backspacing, I'd need a +25mm offset on a 9" wheel, but feel that it might be a little too much positive offset and cause me to rub on the UCA, so I'm leaning towards +18mm.
02-05-2017, 01:16 PM
Some Wheel Width Math
