How to Measure for Bigger Wheels and Tires - Size Matters

Measuring a car for proper wheel and tire fitment

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It’s pretty safe to say that wheels and tires can make or break the styling of a car. You can have a clapped-out rusty turd with a nice set of wheels and it will look pretty cool. On the opposite side, a bad set of rollers can make a fully restored and painted showpiece look less than spectacular.

There are two major factors to consider when choosing new rollers for your ride—style and fitment. Since style is such a personal preference, we can’t tell you what you might like. As for fitment, that is where we can offer some knowledge. Rims and tires, even on the cheap end, are still expensive. Trust us when we tell you ordering new wheels and tires, only to have them rub, hit, or flat out not fit is a bummer.

Now if your car is pretty much stock, then there will be ton of forums where you can do research and find a common fitment for your car. What if you have modified things, then what? Well, you best educate yourself on how to measure the car to find the right specs for your new rollers to come.

That is exactly what we plan to teach you here, since we had to go through the entire process to get the right rims for our ’56 Bel Air. We recently upgraded the suspension with Heidts components, including its independent rear suspension, narrowed tubular arms, and coilovers up front. All these new goodies moved things around enough that we are not confident in letting the Internet tell us what we need. We know we want an 18-inch diameter rim, so our Nitto tire selection is the best, but we didn’t know much more than that.

We contacted B-Forged, a brand of Billet Specialties, to ask what was needed when ordering a set of the company’s modular three-piece rims, and what we got back was a pretty extensive ordering form with all kinds of required measurements. Since this form covers just about every parameter necessary, we are going to show you how to find all these measurements with nothing more than a few squares, a tape measure, a couple of clamps, and a piece of string.

Bforged Wheel Suspension Measuring Guide 2/28

1. Here is the order form from the B-Forged division of Billet Specialties. It’s pretty extensive and once completed will give you and the wheel manufacturer a clear path to getting the right size and spacing on the car. No matter whom you are going to get rims from, it’s a good idea to have all this information before ordering to prevent a costly mistake.

1956 Bel Air Wheel Tire Measuring Bforged Order Form 3/28

2. Here are the simple tools we used to get all the measurements. It’s basically just a selection of squares, two rulers, a few clamps, a set of calipers, and a homemade plumb bob. Preparation is the key for accurate measurements. Make sure the vehicle is level and the suspension is at ride height before measuring. (Jacking up only one side and using the plumb-bob will change the measurement you are getting.)

1956 Bel Air Wheel Tire Measuring Ruler 4/28

3. The first measurement we need to get is mounting surface to first obstruction on the frame side. We clamped one of the rulers to the studs with it flat against the rotor. We positioned the ruler so it protrudes 9 inches from center to emulate one half of the 18-inch rim diameter. Then we clamped one more ruler to that end and rotated the rotor till it made contact with something. At this diameter, our first point of contact was the tie rod end, which was 51⁄2 inches.

1956 Bel Air Wheel Tire Measuring Distance 5/28

4. The next thing to find was the distance from the mounting surface to the fender. To do this, we taped our string and washer combo to the fender. Then we used a square to find the distance...

1956 Bel Air Wheel Tire Measuring Ruler 6/28

5. …which came out to 63⁄4 inches.

1956 Bel Air Wheel Tire Measuring Calipers 7/28

6. Our digital calipers made measuring the hub diameter simple. We got 2.65 or 213⁄20, but the closest yet larger common fraction is 221⁄32 inches.

1956 Bel Air Wheel Tire Measuring Brake Diameter 8/28

7. The square made quick work of finding the brake diameter, which is done by measuring from the top of the caliper to the center of the hub and then multiplying by 2. We have a measurement just shy of 71⁄2 inches.

1956 Bel Air Wheel Tire Measuring Rim 9/28

8. To find how much the caliper protrudes past the mounting surface, we clamped one square, making sure it was flat on the surface of the caliper, and then used another to measure the distance as shown. This ended up being 9⁄16 inch. These last two measurements are crucial to getting proper caliper clearance so the inside of the rim doesn’t hit the caliper.

1956 Bel Air Wheel Tire Measuring Hub Length 10/28

9. Now for the hub length. It was pretty simple: Just place the ruler and read it. Most hubs are much more shallow than the Wilwoods, but these are 23⁄4 inches. This measurement lets the wheel manufacturer know if the center cap will fit.

1956 Bel Air Wheel Tire Measuring Sidewall 11/28

10. The last one in the front was fender height to center of the hub. Again, a large square made this pretty much cake and showed we have 133⁄4 inches. This tells us that with 9 inches of rim (half the diameter), we have room for 43⁄4 inches of tire sidewall. This will help us pick the correct tire od (not that we will need that much sidewall on this low-slung car).

Wheel Clearance Diagram 12/28

11. The last thing you will want to provide your rim manufacturer is a brake system diagram if it’s available. Wilwood has these for all its systems, so all you need to do is go to the Wilwood site and search with your system’s part number. If you have factory brakes, then you probably won’t need something like this.

1956 Bel Air Wheel Tire Measuring Rear End 13/28

12. The rear follows the same procedure as the front. Ours is a bit different than most, as we have the Heidts independent rear end setup. It’s stock width, but because of the control arms and other stuff, we have more limitations than a straight axle. Here we see the first contact point with an 18-inch rim, which is a lower control arm pivot bolt. We found this bolt was 53⁄8 inches from the mounting surface. If we upsize the rear rims to a 20-inch diameter, then our first contact would be the actual framerail.

1956 Bel Air Wheel Tire Measuring Plumb Bob 14/28

13. Mr. Makeshift Plumb Bob was used to find that we had 65⁄8 inches till we hit the rear fenderwell.

1956 Bel Air Wheel Tire Measuring Cv Shaft 15/28

14. The rear hubs are a bit more standard but still different than a normal rear end since we have the CV shaft hardware protruding out. The main part of the hub was 13⁄16, while the CV shaft was 11⁄4 inches.

1956 Bel Air Wheel Tire Measuring Arches 16/28

15. Fender height to center of hub was a bit less in the rear because the ’56 has low swept arches. We have 93⁄8 inches.

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