Considering running 18’s - what fits?

[Nick_R_23]

I have a 1986 GMC C1500 LWB that’s lowered 4.5/6” on stock rally wheels. I ran across a decent deal on a set of 18” wheels that may potentially fit on the truck. It has 235/70R15’s on it now, and the new wheels would be staggered, but I’m waiting to hear the exact specs on them. The tires are 245/55R18 and 295/45R18, so diameter would be very close and I know the widths would be a sure fit. For those of you that have went the 18” route, what specs did you end up with for your wheels to fit? I don’t want any modifications or rubbing.

[Nick_R_23]

Alright, so the specs on these are 18x9.5, ET12 on all four wheels.

[Nick_R_23]

Nobody knows if 18x9.5 with 5.75” backspacing will fit on all four corners?

[mr48chev]

the 18 isn't an issue but the 9.5 is going to put part of the tread of the tire outside the wheel well unless the wheels are offset to the inside.
It isn't the wheel diameter near as much as the rim width and the offset that you have to deal with.

[Nick_R_23]

That’s really what I’m having trouble finding is the offset specs. I’ve found a few older posts with 9.5” rims on the front, but not a single one lists the offset or backspacing that was used.

[geezer#99]

There’s some info here to figure out your bs using a chunk of angle iron and reddy rod.


https://www.hotrod.com/articles/ccrp...heels-fitment/

Or you can buy a nicer tool to be more accurate.


https://www.speedwaymotors.com/Percy...rite,6761.html

[SCOTI]

You can also measure your current wheels installed & do calculations off of them.

[Nick_R_23]

Main problem is that I’m away for work for a while and don’t/haven’t had access to my truck for a while. I’d rather not guess on a fitment as wheels aren’t cheap.

However, since nobody seems to know for sure, I tried to do a little more research. Let’s see if you all agree with what I came up with.

I found a 15x8 rally wheel has 4” backspacing (and also 4” frontspacing). These 9.5” wheels have 5.75” backspacing (and 3.75” frontspacing). This means the new wheel will sit inward only .25” on the outside, but 1.75” inward more on the inside. However, with a 1” wheel spacer, it would now center the backspacing like stock (with 4.75” on each side now), but the wheel would be 3/4 wider than stock on both the inside and the outside of the wheel.

So the big question would be, do I have enough room on both the inside and outside of the wheel to add an extra 3/4” width each direction over a stock 15x8?

[geezer#99]

Originally Posted by Nick_R_23

Main problem is that I’m away for work for a while and don’t/haven’t had access to my truck for a while. I’d rather not guess on a fitment as wheels aren’t cheap.

However, since nobody seems to know for sure, I tried to do a little more research. Let’s see if you all agree with what I came up with.

I found a 15x8 rally wheel has 4” backspacing (and also 4” frontspacing). These 9.5” wheels have 5.75” backspacing (and 3.75” frontspacing). This means the new wheel will sit inward only .25” on the outside, but 1.75” inward more on the inside. However, with a 1” wheel spacer, it would now center the backspacing like stock (with 4.75” on each side now), but the wheel would be 3/4 wider than stock on both the inside and the outside of the wheel.

So the big question would be, do I have enough room on both the inside and
outside of the wheel to add an extra 3/4” width each direction over a stock 15x8?

Your numbers are skewed.
An 8 inch rim is actually 9 inches wide from edge to edge. So with 4 inch bs you have 5 inch front space. Back spacing is measured from the edge to the wheel mount surface.
When you figure measurements remember tire cross section (Bulge)which generally is 1 inch wider than the rim.
And then you need 1 inch clearance beyond edge of tire to anything it might rub on.
It gets real complicated.
Much easier to make or buy a tool like I linked and be accurate.
Some might think an 80 buck tool is a waste of money but you’ll use it more than once.this won’t be the only time you try to stuff your wheel wells with lots of rubber.

[SCOTI]

Originally Posted by Nick_R_23

Main problem is that I’m away for work for a while and don’t/haven’t had access to my truck for a while. I’d rather not guess on a fitment as wheels aren’t cheap.

However, since nobody seems to know for sure, I tried to do a little more research. Let’s see if you all agree with what I came up with.

I found a 15x8 rally wheel has 4” backspacing (and also 4” frontspacing). These 9.5” wheels have 5.75” backspacing (and 3.75” frontspacing). This means the new wheel will sit inward only .25” on the outside, but 1.75” inward more on the inside. However, with a 1” wheel spacer, it would now center the backspacing like stock (with 4.75” on each side now), but the wheel would be 3/4 wider than stock on both the inside and the outside of the wheel.

So the big question would be, do I have enough room on both the inside and outside of the wheel to add an extra 3/4” width each direction over a stock 15x8?

Ahh . . ….. Not having things in front of you does make it difficult to accurately measure. The issue is when others declare something 'clears just fine', it's doing so on a different truck w/possibly a different group of parts & from a different perspective. What one guy is ok with, another may not care for.

It sounds like you're buying 'off the shelf' wheels vs. custom spec (choosing the exact BS/offset) based on the wheel size/BS listed. I really suggest waiting until you can measure in-person. It's a hard choice to have to wait but a better choice unless you can easily buy/return the wheels until you get what you want.

Most guys like the 15x8 w/4"BS factory look. The 9.5" size you listed will be tucked in more but you'll adjust for that w/spacers. That means testing lock-to-lock turning to verify clearance up front because they're still going to be closer to the suspension parts (farther-out because of larger wheel diameter but still farther in-board as far as distance vs the current wheel).

I'm guessing your current set-up clears the fender lip. Your proposed 9.5's would be .750 farther out vs your current set-up. Does it clear by that much?

Now lets move to the rear.....

A 10" wheel w/5" of back spacing is centered in the wheel well & has about the same clearance inside & out from contacting sheet metal. A 10" wheel w/5.5BS starts getting real close to the inner tub.

If your current 8's clear fine, the proposed 9.5's are 1.75" farther in-board vs the Ralley. The 1" spacer here will keep the wheel from buzzing on the inside @ the tub.

In theory (math), they'll clear F&R. They'll have the 'look' that a FWD does or like the late models do w/minimal lip on the outer edge vs. the current steel wheels you have.
The current track width phenomenon (front slightly wider vs rear) might be more pronounced as well.

[Nick_R_23]

Yes, I found some off the shelf wheels that I think would be a great look. I usually do stick with the classic 15x8, but I thought I might switch it up this time. They’re far cheaper than new, but not cheap enough to pull the trigger on without knowing, since they wouldn’t be easy to resell up here.

The current setup (15x8, 235/70R15, 4.5” drop) clears excellent, but it’s honestly not that much tire up front. My previous truck was the same, except with a 255/60R15 tire, and I never had any clearance issues - I just don’t remember how much clearance the rim/tire had in relation to the fender. I am also taking into account that the front track width grows when lowered too.