Wheel Fitment 101

The single most asked question in my inbox. “What is the widest wheel and tire I can fit on my truck?!” We have built a number of trucks now with wheels up to 12” wide in the front. It’s not as uncommon now as it used to be. About 7 or 8 years ago I was trying to build my yellow and white 78 C10 into a competitive auto cross vehicle. By simply watching the fastest cars autocross, they all had one thing in common, a wide front tire. One underlying fact was that Shawn and I had no money. We had two very small kids and a business that was new and barely generating profit. I remember driving back from Autocrosses with them and saying, “If I only had more tire I could be more competitive!” As naive as I was, that was at least the starting point to getting faster. I needed a tire that I could work with. And if I only had a 275 wide front tire, I could only go so fast. So I saved up money to have my Forgeline wheels rebarrelled and I bought a set of BFG Rival 335/30/18.

I had a bolt on suspension system at the time and had no idea if it would “fit”. I got the new barrels for my wheels, put them together and mounted my new front tires. I ended up having to run a 0.5” spacing to push the wheel out enough to not get into the lower control arm. But even at that, I had to toss the mandrel bent sway bar and cut the endlink tab off of the lower control arm. Then I installed a spined front sway bar and made my own endlinks. Even then the tire would slightly touch the arm, and with my steering box, I could only limit travel by making a steering stop where the spindle would hit it on the lower control arm. I wanted to give this foreword because my definition of “fitting” and yours could be much different! I had to do a ton to get them to “fit” and that’s not even talking about how I had to cut the inner structure of the fender out and roll the crap out of the fender lip.

When I start working with a customer on a set of wheels and tires for their vehicle, the first thing that I need to know is their WMS width front and rear. “Wheel mount surface” width is the distance from where the wheel mounts to that mount location on the opposing side. On a straight axle, that measurement is easy. You just bolt the rotors on and measure that total width. On an independent suspension it is a bit more difficult. We need to know the width at ride height. Most aftermarket suspension manufacturers can give that information to you but if not, we can measure. To get an accurate measurement, the vehicle has to be at ride height. You will need to either be able to set the vehicle on cribs and have access to the measurement or remove the coil springs and compress the suspension to ride height with the wheels off. The measurement also needs to be taken at the hub center so you don’t take camber into the equation. Also pay close attention to make sure that the wheels are not turned and there is no excessive toe.

Now that we have that out of the way, it's pretty easy from here. Being able to order the perfect width of wheel and back spacing (or offset) is the goal and will be the last bit of information to enter into the equation. The next dimension that we need is what the overall width can be. That width is from outside to outside of the wheels. Again, on a solid axle rear this is extremely easy so let’s measure the back of the truck. What I do is tape a plumb bob on the outside of each bed side and take the total outside width. Then I deduct the thickness of the bedside lip, and how much gap I want between the wheel and the bed. Below is an illustration showing those measurements.

Now that we have the WMS width and the total width available, we can see if the width of the wheel that we want will fit. The first thing to know is that a wheel’s width measurement is taken from the inside of the bead. The true width of a wheel is actually 1” wider than stated. So a 12” wide wheel is actually 13”, an 11” wide is 12”, and so on. To keep this simple, let's look at the rear wheels since we don’t have to take turning into the equation. I have made another illustration showing the WMS, total width, and an 11” wide wheel with a 5” back space. If we want to push our wheel out to look the best, we will start by subtracting the WMS from the total width. Then that number divided by two will give us the amount of wheel that will stick out from the mounting surface. You can see that we will have 7” of wheel sticking out. If you are going to order a custom set of wheels, you can play around with finite backspacing to get the wheel exactly where you want it. But if you are looking at off-the-shelf wheels, you need to look and see what wheels are offered. If they do not offer an 11” wide wheel with a 5” backspacing, then you have to calculate what they do offer to see if you would be happy with it.

Lets say that they only offer an 11” wide wheel with a 6” backspacing. Working through our equation with that information, it would tell us that we would have only 6” of wheel sticking out from the mounting surface. For me having a wheel set in another full inch is a deal breaker. Let’s go a little further saying that they do offer a 12” wide wheel with a 6” backspace. This wheel would put the outside back where we liked it but would extend another inch toward the inside of the vehicle. Take that information and go back to the bed to see if you have room for the wheel in the wheel tub. **Side bar… These trucks can’t fit a 12” wide wheel in the rear, however we do sell wider rear wheel tubs, wink wink. **

Working on fitting the front wheel is the same process but you have to consider the width of the wheel while turning lock to lock, and under suspension compression. You can start by doing the same thing as we did on the rear to measure for what total width is available. From here you really need to have a wheel and tire to measure off of. There are tools on the market which make this process extremely easy. Pictured is a wheel fit tool offered by Verified Fabrication. This tool is designed to allow you to mount your tire and play around with backspacing. If you want to run a 315 wide front tire, you can find a worn out tire online or simply purchase one tire and check fitment before ordering a full set. These tools hold the tire still, allow for simple backspacing adjustments, and will let you see how close the tire will get to the frame, suspension, or fender while turning. However you don’t have to spend the money on this tool unless you absolutely want to. I have come to find that I like using an existing wheel and tire to take my measurements off of. Even if it's nowhere near the size you want, you can still turn lock to lock and measure from that tire. The only caveat is that you MUST know the exact width and backspacing of the wheel you installed. If you guess it will throw off all of your calculations.

A variable to consider when order wheels is the center bore measurement.

Lastly, don't ever rely on backspacing measurements shared online. Nearly every time we post online, showing a truck with a cool set of wheels, someone is always wanting to know the width and backspacing. What they don’t know is that most of our trucks are built on an aftermarket chassis with WMS widths that are much more narrow than stock. Also, C10’s had different rear end widths depending on the year. Same for the front suspensions, different spindles and brake kits affect front end MWS width. Even what someone thinks fits great may not be your same opinion. I have had first hand experience receiving backspacing information from chassis manufacturers then once I double check, it is nowhere near what I would want to order!

Hopefully now you understand the basics of what it takes to pick your wheel width and backspacing. If I have not covered something that you want to know or need any assistance, please shoot me an email.

jesse@levelsevenmotorsports.com

Jesse Vaughn

Photos courtesy of: Level 7 Motorsports Forgeline Motorsports Tire Rack