Correct Rim Size (Backspacing/Offset)

Description showing Negative Offset

Offset and backspace are two words that essentially describe the same concept although they describe different aspects of that concept. Think of it like this: when offset increases (0 to +12.7 for example) backspace increases (4.0" to 4.5") and when offset decreases (0 to -12.7 for example) backspace also decreases (4.0" to 3.5".) The two are directly related; more specifically they share a direct correlation.

Offset  describes the distance (often measured in millimeters) the mounting point is away from the "centerline" of the rim (the exact middle of the rim.) Offset can either be positive (+) or negative (-); positive meaning further from the back edge of the rim and negative meaning closer to the back edge of the rim. Backspace is the amount of space (usually measured in inches) between the location the rim mounts to the car and the back edge of the rim. Backspace can never be negative.

Measurements for rims aren't entirely accurate for the purpose of calculating offset and backspace. An additional 1" needs to be added to the width of the rim to derive a correct calculation. For example, a 15x8 rim is really 9" wide from outer edge to outer edge. Once the actual tire is mounted, the overall width will likely be even wider (after adding in the bulge of the tire over the rim.) Once you've taken the factory rim size (8" for this example) and added 1" (8+1=9), divide the number by 2 in order to find the cenerline of the wheel; 9/2= 4.5. If the offset for your car is 0 (center of the rim) then the backspace is 4.5. If the offset is +25.4 (or 1") then backspace is 4.5" + 1" or 5.5". Inversely, if the offset is -25.4, then your backspace is 4.5"-1" or 3.5".

In order to figure out offset, you'll first need to know what the measurement for backspace is by using a straightedge and a ruler. Lay the straight edge across the back of the rim (without a tire mounted) and using a ruler or tape measure, measure the distance from where the rim mounts to the brake drum or caliper to the bottom edge of the ruler. This distance is the backspace. Next you'll take the measurement from the previous calculation for the center of the rim and subtract it from the backspace you just measured. If the number is negative, you have what are commonly referred to as "deep dish" rims.  In the case of a 15x8 rim for a 1965-70 Mustang, this measurement should be 4.5". Since 4.5 is exactly half of the overall 9" of the rim, offset is 0 meaning that the mounting point sits in the exact middle of the rim (middle option in the picture below.)

Positive, Zero and Negative Offset

In the case of my 1966 Mustang, I have a rear differential from a later model mustang which means that it is an extra 2" wider than the stock 65-66 differential. This causes the axles to sit out 1" further on each side of the car. I cannot use stock rims because the backspace= 4.5" and with the added 1" in the differential, I need rims with backspace of 5.5". Adding an inch to the backspace will push the offset out towards the fender making it 1" or +25.4mm. Finding 15x8 rims with those measurements hasn't been easy.

I would really like to use Torq Thrust II rims on the car, but I need to see if they manufacture them. I found a website that shows them in 15x8 with 5.25" backspace so I need to measure the space in the rear wheel well to see if that will work.

Torq Thrust II Rim

I set a straight edge from the edge of the fender to the ground in order to see how much room I had before the tires were actually sticking out of wheel well. I do realize that there is a "lip" that takes up about 1" of the inside of the wheel well, but I will be tucking the lip under the fender for added room.

Space between Brake Drum and Quarter Edge

The space between the face of the brake drum and the outside edge of the quarter panel is 4.5" which is cutting things very close with the rims I will be putting on. The distance between the face of the brake drum and the outer edge of the quarter panel on a stock 1965-66 mustang is 5.5". The rims I will likely purchase have an extra 0.75" of backspace (increasing stock backspace from 4.5" to 5.25".) The total rim width of an 8" rim is 9" so when subtracting the backspace of 5.25" from 9", the amount of rim that extends past the face of the drum is 3.75" (well within the 4.5".) However another factor to take into account is the distance the actual tire sits beyond the rim. When measuring my brothers 235 60R14 tires using the straight edge, the tire bubbled 0.75" beyond the rim. Now the total amount of rim and tire that sits past the face of the brake drum is 4.5". Like I said, its cutting it close, but I don't have much of an option with rims and its only cutting it close. The part of the tire that is closest to the quarter panel (the top) is shaped much like a parenthesis; it starts small at the top, balloons out and then tapers back down to the size of the rim.

I called my buddy who works for a national tire chain and he said that the Torq Thrust II rims in 15x8 with 5.25" backspace have to be custom made from two pieces and run $230 with his discount. The two front rims in 15x6 with 3.5" backspace he can get for $150 each. With tax I'm looking at spending around $860 on rims alone. It's time to start saving again.

UPDATE 07/16/11: I bought Torq Thrust 2.1 rims from wheelsforless.com since they were on sale from $224 down to $190 each and I paid no shipping or sales tax! In total I paid $769.91 for four 17x8 rims. The backspacing on the rear rims are 5.48" (close enough the the 5.5" needed) and the fronts are 4.81" (most go 4.75"-5.00" with 17" rims.) They should arrive sometime in the next few weeks at which time I'll see if my friend Shawn can hook me up with some tires. The difference between Torq Thrust II's and 2.1's is that the 2.1's are one piece rims while the others are two piece rims. They look nearly identical to each other.

Torq Thrust 2.1 Rim