Have you ever wondered how some people can fill the fenders of their Chevys full of rubber (particularly late-models with all sorts of "intrusions" inside the factory wheelwell), without resorting to major re-constructive surgery (i.e.: tubs, narrowed frame rails, narrower-than-stock rear axles, etc.)? Sure, you can carve up the wheelwell, but the first thing to do is to figure out just how much "tire" can actually fit inside the existing well-without creating interference on the inside.
To measure the available room...
To measure the available room in the wheelwell, the first order of business is to create a "plumb bob." As you can see, it doesn't have to be fancy-a piece of string with a weight on one end will work (we used a low-tech lug nut for a weight). A dropped "plumb bob" affixed to the inner wheelwell lip works as a reference point for taking measurements.
If you haven't gathered it by now, selecting the right wheel backspace (or "offset") for wheels is critical on two counts: (1) Sliding the biggest wheels and tires into the smallest space and (2) saving you the most amount of money. Adding big rubber in a Chevy that isn't tubbed isn't a problem, but it can provide more than its fair share of headaches. In this case, one mistake on the backspace dimension will mean the wheels and tires don't fit. What about jacking the back of the car up? It's usually a waste of time and traction. We should also point out that it's no secret that most shops simply won't accept an exchange if the wheels show any signs of mounting (can you blame them?).
Wheels are available in a wide array of dimensions. For example, you might be able to purchase a 15 X 10-inch wheel with backspaces that range from 2 inches all the way up to 6 inches. This means that the tire can either be tucked-up under the stock fender of your car (good) or it can hang out in the breeze (not so good).
In order to determine the...
In order to determine the tire bulge, flip a mounted OE-type wheel and tire over so that the brake drum pad mounting flange is facing up. Place a straightedge across the face of the tire and measure down to the mounting pad. Anything that's straight will work-even a carpenter's level such as the one shown in the photo. This figure is the OE backspace with the tire "bulge" included. Repeat the process with a shorter straightedge over the wheel lip only. This is the true wheel backspace dimension (less tire bulge). The OE wheel measurement will only give you a baseline of the available room. In most cars, you'll be able to add more backspace, which in turn allows you to run wider wheels and tires.
Quite frankly, there are a few ways to determine rear wheel dimensions. Some rely upon hit and miss tactics, but the best method is a bit more complex. It involves taking some careful measurements to put all of the dimensions into perspective. By using this method, you can easily calculate the required backspace dimension, and of course, the maximum size of tire that will fit into your existing wheelwell. Here's the drill:
First, locate a wheel with a backspace/offset dimension that's as close to stock as possible. Flip a mounted OEM wheel and tire over so that the brake disc/drum pad-mounting flange is facing up. Place a straightedge across the face of the tire and measure down to the mounting pad. This figure is the OE backspace with the tire "bulge" included. The dimension without the tire mounted is the true wheel backspace. The difference between the "mounted" and "dismounted" numbers will give you an indication of the clearance required for the tire sidewall "bulge." Consider this your "baseline" backspace dimension.
You're not done yet. Most late-model Chevys can swallow a bit more backspace than stock (this also works if you're messing with rearend housings that have been narrowed slightly). You need to take some careful measurements to put all of the dimensions into perspective (you can make a few rough sketches to help).
In most Chevrolets with wider-than-stock wheels, you will find that you can increase the backspace by at least 1 inch from the OEM combination. Keep this in mind when purchasing wheels. Another problem you might run into when "stuffing fenders" is the lip that intrudes into the well. The "bulge" of the tire often smacks the 90-degree lip at the top of the well as the suspension goes through its travel. The solution is simple: The upper 90-degree section of the wheelwell lip (found inside the curbside of the fenderwell), has to be cut and bent inward, which in turn creates a smooth transition from outer body sheetmetal to inner wheelwell.