You’ve decided to buy new wheels for your car. If you’re buying them for your VR-4 or Stealth TT, you probably know that you need to be knowledgeable in the way wheel fitment works because it is difficult to find wheels that fit the VR-4 properly. The problem is that, because many people do not understand backspacing and offset, they rely on the numbers that other people give them. Sometimes this works, but many times it does not. You end up with a wheel that doesn’t fit. A few simple calculations, however, can save you this trouble.

Before I move on to the complex and important terms, let me define a few simple ones that you probably already know, just to avoid confusion:

• Wheel inner face: This is the side of the wheel that faces inward, towards the strut.

• Wheel outer face: This is the side of the wheel that you see (the spokes).
• Wheel width: This is the distance from the inner to the outer face of the wheel. It is not to be confused with diameter.
• Wheel hub: This is the part of the wheel that the lug nuts go through.
• Wheel hub face: This is the surface that mates to the hub on the car’s suspension.

Backspacing

The term backspacing refers to the distance between the wheel hub and the inside face of the wheel. To measure it, place a ruler or a piece of wood across the inside face of the wheel. Then measure the distance from the wood to the hub. This is a critical measurement because it tells you if the inside lip of the wheel will interfere with the strut. Visualize the wheel going onto the car. The greater the backspacing, the further the inner lip of the wheel will go before the wheel’s hub touches the car’s hub. Too much backspacing and the inner lip will contact the strut before the hubs meet, making the wheel unusable. Click here for a diagram and imagine the car’s strut and suspension arms to the right of the wheel. The backspacing is indicated center-right in the picture.

Offset

This is the term that often confuses people trying to buy wheels, but it is simply a relative measurement of a wheel’s centerline and a wheel’s backspacing. In this graphic, couresty of the good people at Discount Tire Direct, you’re viewing the wheel from the front—meaning it is as if you are standing directly in front of the car looking at the tire tread. The right side of the wheel is the outside face (the spokes) and the left side is the inside face. The centerline is the exact middle of the wheel from this perspective. For example, if you have an 8” wide wheel, you measure 4” in from either the outside or inside face to determine the centerline.

Offset is simply where the wheel hub face is relative to the centerline. In the graphic, the wheel marked “zero offset” has the wheel hub face precisely on the centerline. In our 8” wide wheel example, a zero offset would mean that the backspacing is 4”. Imagine putting this wheel onto a car: 4” of it have to go past the car’s hub, over the brakes, and in towards the strut.

Now this is where it can get a little confusing. A zero offset isn’t an extreme; it’s not one end of the range of possible offsets. In fact, it’s right in the middle. Positive offsets mean that the wheel hub face is moved (or offset, hence the term) away from the centerline towards the outside face (the spokes) of the wheel. Negative offsets mean that the wheel hub face is moved away from the centerline towards the inside face of the wheel.

Consider the graphic again. In the example marked “positive offset” you can see that the wheel hub face is no longer on the centerline; it’s moved to the right, towards the spokes. Now let’s do some hypothetical measurements to our 8” wide wheel again. We already said that with an offset of zero the backspacing would be 4”, meaning that you need 4” of clearance for the wheel to actually fit on the car and not hit the suspension. Let’s now assume that the same wheel has a positive offset of 25mm (or approximately 1”). The backspacing is going to increase from 4” to 5”. That means that the inner lip of the wheel is going to be an inch closer to the suspension and the outer lip of the wheel is going to be an inch closer to the wheel hub face. If you had a tight fit with 4” of backspacing, you might have interference with 5”—even though the wheel width is still the same, 8”.

With a negative offset the opposite is true. If our 8” wide wheel had a negative offset of 25mm, the wheel hub face would be offset towards the inner face of the wheel. Backspacing would decrease from 4” to 3” and you would have even more space between the strut and the wheel. Some “deep-dish” Lamborghini wheels have negative offsets.

Now let’s look at how offset and backspacing come into play when you’re trying to put a wider wheel on your car. A stock VR-4 wheel is 8.5” wide, with a positive offset of 46mm (+46mm is how this is noted). To find the backspacing without using a ruler, we find the centerline: 8.5” / 2 =4.25”. Then, because the offset is positive and the wheel hub is further towards the outside face, we add 46mm (or 1.81”) to 4.25” and get 6.04” of backspacing.

But now you want to go up to a 9.5” wide wheel (this is what I use on my VR-4). Use the same simple math to find the centerline and the backspacing for a 9.5” wheel with zero offset: 4.75”. Now find out what offset would give you the same backspacing as the original wheel: Subtract the location of the centerline of the new wheel (4.75”) from the total stock backspacing (6.04”): 1.29”. Since offsets are almost always listed in millimeters, convert by multiplying by 25.4: 32.7. Therefore, in order to have the same backspacing with a 9.5” wide wheel—meaning the same wheel-to-strut clearance, effectively—you need an offset of +33mm. Want more clearance? Go with a lower offset. Think you have a little space to play with? Then you can use a slightly higher offset. This is what I did on my VR-4. With the stock wheels, there is a little less than a half-inch of clearance from the tire to the strut. So when I bought my 9.5” wide wheels, I felt safe with a +36mm offset. I was making the clearance from wheel to strut a little less, but not enough to matter. Sure enough, they fit fine.

Brake Caliper Clearance (or “Frontspacing”)

Everything I’ve explained so far applies to cars and wheels in general, but this section is specifically directed at VR-4 and Stealth TT owners, as well as owners of any other car that has very large and oddly-shaped brake calipers. Remember that backspacing refers to the distance the wheel protrudes into the wheelwell and that there is a limit because the strut cannot be moved. There is a parallel situation on the other side: the caliper has to fit in the space between the spokes and the centerline. Imagine for a minute our hypothetical 8” wheel with an offset of +75mm (or 3”). The backspacing would be huge (7”) of course, but on the other side the caliper has to fit inside, at most, an inch of room.

With large calipers like the stock ones on the VR-4, you need to size your wheel such that you have a lot of “front” space—room for the caliper. Wheels with spokes that curve inwards or wheels with significant lip often cause VR-4 owners problems because the backspacing is correct but the caliper, which extends quite far out away from the car’s hub and towards the outside face of the wheel, hits the spokes. You can measure this distance on a VR-4 or any car by putting a ruler on the outer face of the caliper (where it says “Mitsubishi” on a VR-4 caliper) and then measure the distance between the ruler and the car’s hub. However, it can be difficult to judge the spoke shape of aftermarket wheels simply by looking at a picture.

That’s why it’s best to choose wheels with little or no lip and with spokes that do not curve inward. The 5Zigen FNO1R-C is a very popular wheel for the VR-4 in part because there is hardly any lip and the spokes curve outward, removing the risk of caliper interference.

Note: The offset of some aftermarket wheels is changed by moving the entire spider (spokes and hub) relative to the ring of the wheel. In this case, the “frontspacing” will not change with offset. In these situations it is more difficult to determine if the wheel will clear the caliper, and test-fitting may be the only option.

Summary

If you can understand backspacing, offset, and the relationship between the two, you can select properly-fitting wheels for virtually any car. Remember that offset is a relative measurement that is derived from the wheel width, the centerline, and the position of the hub. An offset of “zero” doesn’t indicate the extreme of a range, but the middle. A given wheel width will usually have several available offsets, and a given offset will result in different backspacing if the overall wheel width is changed. You need to imagine the wheel fitting inside the wheel well, going around the car’s hub and brakes and coming close to the strut. The strut, the brake caliper, and the car’s hub face are fixed and cannot be moved; choosing a proper offset and wheel width ensures that your wheel will not interfere with any of these immovable parts.

A quick way to remember this (which you should not memorize in place of understanding the concepts fully): If your wheel has a positive outset, decreasing it will move the wheel out away from the strut. So, if you’re increasing your wheel width, you’ll need a lower positive offset.

If your wheel has a negative offset, increasing it will move the wheel out away from the strut. If you’re going to a larger wheel and need more strut clearance, you’ll need a higher negative offset.