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Chevrolet Wide Tire Install - Fat On All Fours

The Complete Guide To Stuffing Monster Rubber Under Your Chevy

Jeff Smith Mar 1, 1999

Step By Step

This is Duane Uyeda’s killer ’69 Camaro dressed with both 17-inch...

...and 20-inch (right) polished American Torque-Thrust II wheels. Duane will be the first to admit that the 20s really aren’t driveable because of fenderwell clearance problems, but they sure look brutal. These new 20-inch Americans are also available in custom backspacing dimensions. For the 17s, the front tires are Goodyear 245/45ZR17 F1s mounted on 17x8-inch Americans with a 4-1/8-inch back spacing. The rears are 275/40ZR17 F1s mounted on 17x9.5-inch wheels with 4-1/4-inch back spacing. The monster 20s measure 245/40ZR20 fronts on 20x8-inch Americans with 4-1/8-inch backspacing with the rears at 275/35ZR20 mounted on 20x10-inch wheels with 5-1/2-inch backspacing. Whew!

This photo illustrates how dinky the original 15-inch Rally wheels and tires look when compared to both the 17s and new 20s. “Plus-sizing” refers to increasing wheel diameter. In this case, a 17-inch is a plus-two, while the 20s are an amazing plus-five! If there is a downside to 20-inch wheels and tires, it’s that the reduced sidewall allows little cushion to prevent wheel damage from potholes.

Duane used Fat Man narrowed upper control arms on his Camaro to clear the very tall, deep-offset 20-inch wheels. Keep this in mind when contemplating deep offset wheels.

Fitting fat tires is immensely easier if you take the time to build this simple clearance checking tool out of angle iron and threaded rod. Cut a length of 11/2x1/8-inch angle iron 18 inches long. Drill two 7/16-inch holes 43/4 inches apart to bolt the angle iron to the brake hub. We also drilled a series of 1/4-inch holes at lengths varying from 11 to 14 inches from the axle centerline (halfway between the two 7/16-inch holes). The smaller holes represent the varying sidewall heights of different height tires. We used a 1/4-inch threaded with two jam nuts to simulate the inboard and outboard sidewall widths.

We started with the rear axle, bolting the checking tool to the drum brake flange. Note that the axle hub prevents the tool from fitting flush to the drum. You must compensate for this difference in the final measurements. Then we adjusted the length of the threaded rod until it just touched the inner wheelwell. Next, we installed the rod at different “heights” on the angle iron to see if the clearance changed based on tire height. In our case, we found shorter sidewall-height tires offered more clearance. This means you need to be very certain of the tire height as well as the width you intend to use. Be certain that the axle is placed at ride height inside the wheelwell. If not, your measurements will be bogus. Also keep in mind that the tire will go through as many as three or four inches worth of suspension movement.

Next, switch the threaded rod around to check the outboard clearance. In our case, the trailing outside edge of the wheelwell was the tightest. Always measure all four corners of the car. We found that the body on our test Chevelle had shifted to the left in relation to the frame by approximately 3/8 inch! Hotchkis’ Scott Gillman experienced this same situation on his ’64 Chevelle wagon and remedied it by loosening all the body mounts and “bumping” the body to the right until both sides were even.

Checking clearance on the front, the tool wouldn’t straddle the large center hub so we just bolted the angle iron to the hub flange with one nut and a few spacers to clear the disc brake caliper. We found that the front usually hits the sway bar at almost full-lock on one side and/or the rear wheelwell area on the other side of full-lock. Again, make sure the suspension is at ride height. If you really want to do it right, remove the front coil spring and check clearance with the suspension in full-bump to make sure the tires won’t rub.

While large amounts of backspacing are possible even with ’60s cars, be careful that the wheel doesn’t crash into the suspension. Generally, big diameter wheels like 17s or larger will miss the outer tie rod but can hit the upper ball joint. In our Chevelle test case, the 6-inch backspacing wheel just cleared the custom tubular Global West Suspension Components upper control arm. A stock upper arm would not clear this wheel.

The easy way to check backspacing on a wheel is to lay a straightedge across the back of the wheel and then measure from the wheel mounting flange to the bottom of the straight edge. If the tire is mounted on the wheel, this measurement is the combination of wheel backspacing and section width. Without the tire mounted, you need to know how much more the tire will extend beyond the width of the wheel. The widest part of the sidewall is called the section width. This will change by about 0.2 inch for every 1/2-inch change in wheel width. As the wheel becomes wider, so does the section width.

After completely measuring the car, we mounted a brand new set of BFGoodrich g-Force T/Akd 275/40ZR17 tires on a set of Budnik 17x91/2 wheels with 6 inches of backspacing and bolted them on our ’65 Chevelle test car. They fit very tight but look outstanding.

While the 20-inch Americans may be too much on the front for most cars, these killer-tall wheels can work very well on the rear. If there is a downside, the 20-inch-tall wheels make any rear caliper and rotor look absolutely tiny.

What defines an aggressive street car? Paint, a thumper drivetrain, and stance are all important, but it usually comes down to monster tires. For the last 15 years, Pro Street has led the way with a killer look revolving around steamroller rear tires. But that image is changing. If you are one of those Chevy faithful who really likes pushing the envelope, the best way to do that is by subscribing to the Pro Touring concept with fat tires at all four corners.

If there is a lost art amid street machiners, it has to be fitting fat tires. How many times have you seen an otherwise good-looking street car all jacked up to clear big tires? Or, even more common, the sliced and diced sidewalls that come from the tire rubbing on razor-sharp wheelwell openings? Most enthusiasts do little more than take a barely educated guess at rim and tire size and then hope for the best. But, if you are going to spend up to $2,500 on tires and wheels, you should spend a couple of hours making sure the rolling stock fits properly. This story will help you do just that.

Measuring Up

While tire height, width, and rim size are important measurements for fitting fat tires, there are a couple of other measurements that are also critical. Few enthusiasts take into consideration wheel backspacing and the overall tire width when mounted on the wheel when measuring for tire clearance. The next important dimensions are accurate maps of both the front and rear wheelwells to ensure that your tire and wheel will fit properly without interference.

The first step is to build a very simple wheelwell clearance device out of 1-½x1/8-inch angle iron and a length of ¼-inch threaded rod. As you can see from the photos, the angle iron bolts to the brake hub and then uses a threaded rod to accurately trace inboard and outboard sidewall clearance. When taking these measurements, always check that the rear axle is at ride height and not extended to full droop. Use a floor jack to place the axle at ride height. You could also remove the springs to check clearance with the car at various ride heights, but it’s not necessary.

It’s also critical to check clearance on all four wheelwells to ensure that the car sits on the frame squarely. On our test car, we found the body to be shifted to the left relative to the frame, creating a tighter fit on the right outboard and left inboard areas. To remedy this situation, loosen all the main body mount bolts and shove the body in the appropriate direction to center it over the frame.

When checking the front tires for clearance, always move the steering to full-lock in both directions to check for interference. This is where the checking tool really comes in handy. This tool will immediately show you where the tire will hit with the steering at full-lock. On our test car, we found that with maximum backspacing the tire would hit the sway bar in one direction and the upper control arm in the other full-lock position. Since this interference occurs at almost full-lock, and generally at very slow speeds, this may be acceptable. If you are aware of this, you can usually avoid scraping the sidewalls. An additional steering lock stop extension would also prevent the tire from rubbing.

Once the measurements are complete, it’s a judgment call as to how much tire clearance is acceptable. We suggest ½ inch on either side of the tire as a clearance, but we’ve seen successful fat tire stuffs with as little as a 3/8-inch clearance. For rear axle applications, one additional step is to limit lateral (side-to-side) movement of the rear suspension as much as possible. For coil spring cars like Chevelles, Impalas, and third- and fourth-generation Camaros, the best idea is tubular lower control arms by Global West, Hotchkis, and Art Morrison. Our personal experience is these components do an excellent job of reducing lateral movement inherent with stock-type, stamped factory control arms. This is crucial with sidewall tire clearances of less than ½ inch.

For rear leaf spring–equipped cars like first- and second-generation Camaros, the best plan is to use reduced-deflection bushings such as polyurethane, or a set of Global West’s Del-A-Lum bushings to limit lateral deflection. In this situation, the Global West bushings do a better job since they offer little lateral deflection. This is especially useful when negotiating steep driveways when one tire angles up into the wheelwell. Taller tires are especially vulnerable to lateral movement and scraping in these situations. We know of an early Camaro that had 1 inch of sidewall clearance and still rubbed a set of 275/60R15 tires on a not-so-steep driveway. Shorter sidewall tires are less susceptible to this problem.

Once all your measurements are complete, one of the biggest problems is finding a set of wheels that match your requirements. It’s often difficult to find a wheel style you like that will also fit within the narrow confines of what you need. American Racing, Budnik, Weld, and others will build custom offset wheels (within the limitations of the wheel) that can be specified down to as little as 1/8-inch backspacing increments to fit your needs. The best thing to do here is to call the specific manufacturer and see what services they offer. Of course, you can expect to pay a little more for custom-offset wheels than standard off-the-shelf ones.

As far as tires are concerned, keep in mind that the tread is not the widest portion of the tire. Depending upon the width of the wheel compared to the tire, the sidewall width (referred to as the section width) can add ¼ inch or more per side to the width of the tire.

Another factor that is less well known is that tread and section widths vary greatly between manufacturers with the same size tire. For example, we measured a 245/50ZR16 Goodyear, Firestone, and BFGoodrich tire and found that, even when mounted on the same size wheels, the section widths varied by as much as 3/8-inch. Tire heights were all very close, but the section widths varied substantially. This becomes critical when changing tire brands. As you can see, there’s more than meets the eye when it comes to stuffing monster meats under your hot street Chevy. However, if you take the time to carefully measure all the clearances and select the best-fitting tire and wheel combination, it is possible to have that killer, fat-tire look in even a daily driven street car.


Hotchkis Performance
Santa Fe Springs, CA 90670
Budnik Wheels Inc.
Huntington Beach, CA 92647

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