After you understand how width and offset work , you can figure out  where you want your wheels to sit.

You always need a good baseline to base your fitment on. While you can do this with your OEM wheels, I always recommend using aftermarket wheels. Most of us wheels whores will usually do our fitment based on our first set of wheels, or using other cars for examples. Now this doesn’t mean you have to buy a set of wheels to get started, you can just get some wheels from a friend or shop for test fitment, and snap some pictures for reference.

Once you get your baseline down, you need to figure out where you want to wheel to sit in relation to the fender. It’s also very important to understand where the tire is going to sit, alignment specs, ride height and what (if any) fender modifications are needed.

General rule of thumb:

If you’re modifying for function, aka track use, you’re not going to be stretching the tire. Drifters will disagree with this statement, as they’re the only ones who stretch for the track (nothing wrong with that by the way). Track guys will probably want some meats on wheels to increase grip. Their width is going to be important because that’s what’s going to determine the size of tire they’re going to be running. They also need to pay attention to their alignment settings as they’re not going to be running broken axel status negative camber. Ride height and fender work is also important because rubbing will be a big concern with big meats.

Now if you’re going for the Fitment is King crown, you’re not constrained by the above issues. What matters to you is getting your car to sit pretty. Plainly put, your fitment will based on how far you are willing to go to achieve your opinion of perfect stance. Whether it’s tuck, flush or poke, you need to understand what goes into each in order to decide.

Measuring fitment – the goods

You will need a tool to take your measurements. I like to use both tape measures and digital calipers. If you want to stuff the absolute widest wheel you can get in there, you’ll also need to put the car on a lift or jack stands.

Inner

If you are going for girth, read this. If not, skip and move to the next area.

You need to measure the clearance of the inner side of the wheel to anything in there that you’ll hit. Typically the strut is what you’ll hit first, but YMMV, so check around. With the baseline wheel on, find the obstruction, and take the measurement from the inner lip to the place that you’ll likely hit. (You might be able to modify whatever your hitting, remember this is by no means an end all be all guide). Write it down, and save it.

Outer

With your baseline wheel on, you need to need to decide where you want the wheel to sit. There are some very important things you need to take into consideration. They are:

1. Where you want the wheel – battleship sunk? tuck? flush? poke?

2. Ride height – How low are you? How low do you want to be? Can you adjust your height? Coilovers?Bags?

3. Stretched tire – Are you willing to stretch? How much of stretch do you want?

4. Fender mods – Are you willing to roll you fenders? How about pull? How about pulled to all hell? Flares?

5. Alignment – Are you willing to run camber? How much camber? Can you even adjust camber?

After you answer those questions, it’s time to get measuring. Pick the spot you where you want the wheel to sit, and using your tool of choice, measure from the outer lip of your baseline wheel to your desired spot. Take your measurements write it down and save it.

REMEMBER TO TAKE MEASUREMENTS FOR BOTH FRONT AND REAR

Now it’s time for some math.

Using the offset calculator

The easiest way to do this, is to take out all the human error, and use this offset calculator here. There’s still some guess work needed, it’s not a magical number machine. To use the calculator, enter the baseline wheel specs, then take your best guesstimate as to what you think your new wheel will be. The calculator will then spit out what the difference is for both the inner and outer. Play around with the numbers until you find a spec that is exactly or close to the numbers you wrote down earlier. A lot of the time you won’t get the exact specs to your measurements. At that point you will need to decide where you want to give up some of the change. For me, I’ll give up inner clearance any day of the week. As long as my outer specs are on point, I’m a happy camper. If you’re a track guy, you may want to maximize your inner clearance so you won’t have to do as much fender modification. As always YMMV.

Using a pen and a piece of paper

Now, if you want to do the math by hand, here’s some of the basics you need to know. The offset calculator is mos def the easier way to do this. I recommend you use the calculator listed above, but it’s always good to get a idea of the logic going on.

I do everything in MM, so first

Take your width x 25.4mm = width in MM.

Once you have that you can begin playing with the numbers on paper. I usually start with width, then do offset, then go back and forth between the two until I figure it out (I’m by no means the smartest guy here).

Now, when you keep offset the same, and add width, the increase applies evenly to both sides.

For example

1″ increase in width= 1″ x 25.4mm = 25.4mm / 2 = 13.7mm added to both sides (I always round up, so 13mm)

1.5″ increase in width= 1.5″ x 25.4mm = 38.1mm / 2 = 19.05mm added to both sides

2″ increase in width= 2″ x 25.4mm = 50.8mm / 2 = 25.4mm added to both sides

After you do that math, you can start changing offset. Remember that if you add offset (positive offset) it brings the wheel in, or if you decrease offset (negative offset) if pushes the wheel out.  Change the offset, and move the numbers in or out until they are near your spec. Now, you can and change the width again. Then offset again. Then width. Then offset. Then width. Then offset. See why the offset calculator is easier?  I’m sure there’s an easier way to figure out how to get there. Honestly, I got into halfway through geometry in the 8th grade, then when I got to high school, then put me into math 101.  Go figure. After that, I stopped giving a shit lol.

The biggest thing to remember to when calculating by hand is to remember that width is always in 1″ or .5″ increments. The first couple of times I did this, I checked my math only to end up with a weird ass width. Also, the same rule applies as above. Most of the time you’ll need give up either the inner or outer, so pick and choose your poison.

Measuring for a Rim Half / Lip etc  AKA BBS RS / OZ Futura and the like.

This is easy as shit. You want bigger lips? No, not fat injections for your Angelina Jolie’s. Cakewalk shit. From the outer lip, measure in .5″ increments where you want the wheel to sit. When you say “that’s it!” take that measurement (in .5″ increments member?), and order that size rim half. EASY. You can do the reverse for the inners if you need less poke.

The math on the a rim half is easy. For whatever change you do, you only adjust one side (inner or outer) of the fitment, and adjust the offset by width in MM.

Example. You have a 1″ lip on your BBS RS. You want 2″ lips. You measure 1″ outwards from the outer lip. You think, “Wow that’s awesome what are the specs?”

You would add 1″ of girth to the wheel, then either increase or decrease the offset. Typically people add bigger outer lips, so in that example you would decrease offset.

Easy right?

What do I do now?

Most of the time, we work backwards. We think, “Damn that wheel is badass, i want that on my car.” Then we just pick whatever size is available that we think will fit (or that someone recommends), and go with it. Or, we’ll see an aggressively sized wheel, pick it up and try to make it work. Now in a perfect world, every wheel would have endless options for sizes, but a perfect world it is not. Take your new found measurements, and pick some wheels you want. Hopefully they will have something close to your specs. If they don’t, you can plug in the sizes in the calculator (or do it by hand…if you really want to) and see what the difference is. Since you know how all the math works now, you should be compute the small differences in your head. If you’re going for 3 piece wheels, you should be able to do either a 100% custom offset, of they’ll have a selection of offsets that should be within 3-8mm of your specs.

Important things to remember

Remember this is by no means an end all be guide. I’m by no means the smartest guy (or even a smart guy) when it comes to wheel fitment. This is simply my experience from all my cars, helping some of my friends out with their fitment, and working for one of the best wheel shops ever (BIG SHOTS TO MEMORYFAB!). With that said, here are some things to keep in mind. This list will continue to grow as I think of more stuff.

Don’t do aggressive fitment on springs. If you end up rubbing, it might be too bad so sad.

Don’t do aggressive fitment if you don’t want to roll your fenders.

For the outer measurements, a 3-5mm difference is NOTHING. Hell, I’m usually not worried about 10mm difference. A quick roll, or a very slight pull (one you can even see) is all it takes to cure some minor rub.

It takes time to get perfect fitment down. It’s never a bolt on and go situation. Expect to do work son.

Spacers can always be used to push the wheel out if you need. Remember that if you’re planning on running spacers, do the measurements on for the size of the spacer you want. Also note, when using spacers, make sure you have enough thread on the stud/bolt. Safety first!

With the logic of using spacers to push the wheel out, can we shave the hub down to increase offset? Yes and No. Yes you can but probably 3MM tops. is 3MM worth it? Probably not. Can you shave more? Sure why not. Is it safe? No. Do I recommend doing this? No. Should you do it? No. Well my friend said XXXX? No. Don’t do it.

I’ll update this as I think of more.

Additional info:

Caliper clearance

If you’re ordering new wheels, you’ll need to determine which size disk (hub) you need to clear your brake calipers. Typically, standard sized disks are the norm. If you have drums or itty bitty brakes in the rear, you can get away with low disks. If you have massive brakes (front or rear), you will need race disks with high caliper clearance. This doesn’t really have anything to do with fitment, but it will change the way your wheels look in relation to how much dish they have or how concave they are. However, if you some how manage to hit the caliper with your wheels, you’ll need to add a spacer to clear the caliper, which in turn changes the way the wheel sits. Now the spacer could be anywhere from 5mm to 25mm, or you could shave down your calipers. There are too many variables to say what you will need. Do some research.