4.5" of backspace is what is recommended with that exact size tire. You will have to trim some in the wheel well for clearance reasons, according to the instructions.

So basically those wheels are exactly as spec'd by rancho to work.

Also, you CAN run a bigger tire on those wheels should you chose to do so, but you will need to trim more, and if you get too wide you'll need to cut and plate the front body mounts (easy enough to do). Also, you can run less backspace, but again, you'll be rubbing more on the fender liner/flare.


So in short...ignore offset, focus on backspace and tire width and you'll be good.

 

Wheels and Tires are a hassle with all chevy trucks because they require a positive offset. That is why you always see guys with their tires sticking out past the fenders, throwing crap all upside their truck and having to replace the bearings all the time due to the stress that the wheels place on them. When I went to buy wheels and tires, I first tried a "0" offset and with just 235 wheels, (stock on my 2011), they stuck out past the fender by about an inch. It took me forever but I found some 16 x 7.5 wheels with a +40 offset and I threw on some duratrac 245, 75, 16r wheels on them and it looks great, flush with the fender and I'm not stressing out my wheel bearings. I know you want to put the large tires on your rig and no doubt it will look good, but if it were me I would try to find a wheel with either 0 backspacing or a + offset, and I wouldn't go bigger than 265, 70, 17r. Just my opinion, good luck!

 

FYI: zero offset isn't of much value without a wheel width.

A zero offset on a 9" wheel is different than a zero offset on a 6" wheel.
Out of curiosity, what wheel width did your 235's rub with 0 offset? I'm having a hard time finding a width with 0 that looks too far tucked.

The +40 option on the 7.5" wheel you mentioned will tuck the wheel in 3/4" more than a stock Z71 wheel. And 1/2" more than a stock Z85 wheel.

 

Your choice in tire will matter as not all 285s measure the exact same width... I'm extremely close to rubbing the upper ball joint with 285/70/17 duratracs with 8 inch wide wheels with 4.5 inch backspace and I have the rancho lift.


I am currently using 5mm wheel spacers to bring the tire away for the ball joint a little more

 

Thanks guys. If I were to find a wheel with 4.25 backspace, would that give me more clearance from the balljoint or is that the other way around?
Since some of you guys have spacers I'll go ahead and ask, do you have any trouble with them? I basically drive rough, gravel roads with wash ditches all over. If I run spacers I just want to be sure I don't have to worry about them coming loose.

 

ring type spacers like mine are safe up to 1/4 inch or 5mm thick. Just re torque after a few miles of driving after installing them and then every few thousand miles after just to be safe. If you find a wheels you like with 4.25" of backspace you shouldn't need spacers wi the size tire you want to use.

 

That would give you more clearance. I'm running 15x8 with 4" backspace (0 offset), with 33x12.5 tires. I'm not running a lift kit, just a torsion bar crank. From measuring I have about 1/4" clearance from the ball joint if I run a lift kit. Your tires are about 1.25" narrower than mine. So the math says you should have 5/16" of clearance with a 4.5" backspace...more clearance if you run less backspace.

In reality, you'll have more than that. A lift kit on my wheel/tire setup would put the ball joint right next to the widest part of my tire. With your rims being larger, the widest point will be higher up, offering more clearance. Personally I'd go with something with a 4.25" backspace, no wider than 9", 8.5" would be preferable.

Fyi my truck is a z71 4x4 also.

 

Are GM's and Rancho's spindles comparable in size?

 

Right now with my set up and the 7.5 inch wide wheels and the +40mm offset it works out to the following dimensions. 40mm = 1.57 inches, so of my 7.5 inch wide rim 4.82 inches are toward the ball joint and 2.68 inches are toward the outside. If my tires are 245/75/16, then they are 9.6 inches wide, sitting on that rim they would have 3.4 of those inches sticking out and 6.2 inches tucked up under the fender. If I had 0 backspacing (50/50 split) then 4.8 inches would be tucked under the fender and 4.8 inches would be sticking out, that means that my tires would be another inch and a half out past my fenders, thus putting a lot of extra stress on the wheel bearings and the need for a much wider fender flare to protect from mud and rocks. BTW I don't have a lift on my truck. Still if you go with a negative offset I would still only recommend a 265/70/17 tire, the extra width of the 285 would only increase the risk of problems with very little noticeable difference in looks and performance.

 

Custom rims, wheel tire packages for your ride - RIMSnTIRES.com

Online Wheel and Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit

Links with visuals of how a wheel moves when comparing different specs.
According to the calculators, a zero offset is not center of the wheel (such as 4" is not a zero offset on an 8" wheel)

 

Fyi... 4.5 backspace is 0 offset on a 8 inch wide wheel

 

Good links for the calculations, I did them by hand in my post. To sum up my point, I see too many trucks with their tires sticking out, when they could have bought the proper size wheel and had most of their tires tucked up under the fenders. I guess it comes down to the following questions. Will my tires rub on the inside when I turn, or will they rub on the outside when I turn? To solve these problems you will either need spacers or you will have to do some trimming, respectively. With a lift It is easy to just go with a deep dish wheel that will have most of the wheel and tire towards the outside, this will raise your ride and you most likely wont have anything rubbing on the inside when you turn. I see alot of guys do this with their trucks, problem is they have alot of rubber showing and that will flip mud, rocks, etc. up the side of your truck and kill your paint job, also the truck sits with the center of gravity straddled between wheels that are too wide, and that is murder on your bearings.

 

This guy is going to be running lift spindles which requires a certain amount of backspace for spindle/ball joint clearance. Its not so much about wheel well clearance. he's got z71 flares so with 8 inch wide and 4.5 inch backspace he be about even with the flares. Yes the side of the truck may get dirty. No way around it with lift kit spindles unless you extend flares out or use a ridiculously skinny tire and wheel which looks bad on a lifted truck

 

Alright, I just need to decide whether I can find a wheel I like with 4.25 BS or just go with the 4.5 BS one's I've found and deal with very close tire/balljoint.

One more question though, if I get the wheels that are 8.5 wide instead of 8, are the 285's going to look goofy from the extra half inch? I'm just curious.

Doesn't matter too much if they stick out past the flares a little, my truck spends more time dirty than clean anyway haha. This setup will only contribute to that.

You guys rock. Everytime I've asked questions here, I get a response that answers. :salute:

 

If you are going to go with the 285's that is roughly an 11.2 inch wide tire. With that being said your wheel is wide enough that you wouldn't really notice any difference between mounting on an 8 inch rim or an 8.5 inch rim.

 

Bingo!

Let us know what you decide to go with. Also, what tires are you gonna run? Make sure to post up pics!

 

I've got 265's on 8.5" wheels. I would have preferred the wheel to have been 8" for my tire width. It's not bad, but it could look better. You can't go wrong with either one of the two widths.

I'd personally stay with 4.5" backspace over the 4.25" though. Just for personal preference in terms of looks. Too much outside doesn't look good to me.

 

We are talking about a 1/4 of an inch... won't even notice the difference.

 

I have always run BFG All-terrains, and I love that tire. Last summer I tried the Good Year Duratrac, and so far so good. You have the V-8 in your truck so I wouldn't be too worried about losing too much power, however be careful because when you start getting to the larger tires some of them weigh a ton and will cause decreased gas mileage, performance, and extra wear and tear on your truck, remember weight is an important dimension too.

 

Yep that's one of the reasons I'm trying to get aluminum wheels and not steel. Not like I get fantastic mileage anyway. I'm also in the process of switching all my fluids in the truck before the lift goes on.

A tune once I get the lift and tires on will help too I hope.

Oh and GregDAP, your truck is badass.

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Thanks. Yes a tune will help a lot and fix the sloppy transmission. You probably got 4:10s which will help too

 

I do indeed. My plan was to have the lift and tune last year but it didn't come together. Soon though she will finally be the way she should. I appreciate all the help you guys, it's awesome to have a place to get answers from.

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Update: Getting ready to order the wheels for the lift since 1BADI5 pointed out that discount tire has a great sale on their ebay page.

I called Rancho and the tech I spoke with said that 17x9 is the wheel size they use. With the 17x8.5 that I'm going to order, I'll have just a bit more room from the balljoint correct? I know it won't be much but any bit helps. I also understand that our hubs are lug centric, so the larger bore size should not matter.

 

Depends on the offset. You can make a 9" wheel sit identically to an 8.5" wheel. The calculators above will show you examples of how you can change the offset for the same overall value.

 

I checked them out again, thanks. Looks like a 8.5 rim with -6 offset (4.5" backspace) will give 20mm (!?) more clearance. Since that takes into account the tires too it should be fine :th_woot:. It'll just stick out a tiny bit more from the flares.

 

I don't know what the 20mm is your comparing your number to, but you're right about the 4.5" being about right for the Z71. It should sit nicely with the flares (so I understand). I don't have a Z71 myself.

 

To quote the site: "Compared to your existing wheel, this new wheel will have an inner rim which is 20mm further away from the suspension strut. The outer rim will poke out 58.1mm more than before."

Judging by the measurements myself, this should be just fine.

 

I'm just curious and confused.

What is this 'existing' wheel you used as input? A random value from a 9" wheel, or your stock wheel?
Again, just curious. You keep leaving out parts.

Greg has the experience since he is lifted. I'm only curious about the numbers you're using. Trust his input over mine when it comes to clearances.

 

I don't trust those sites too much when it comes to actual fitment and rubbing. Even with 4.5 of backspace Your tires are going to very close to the upper ball joint like the pic I posted