Information from Skulte Performance. Skulte Website
We will use an ’82-92 Camaro as the example.
Offset is the distance from the rim’s mounting surface to the centerline. This Camaro originally came with 16×8″ rims with 0mm offset in the front and a positive 16mm offset for the rear rims. Positive offset means the mounting surface is moved towards the outside (away from the center of the car) of the rim. Negative offset is the opposite. Backspacing Backspacing is the inside depth of the rim. It can easily be measured by putting a yardstick across the backside of the rim, and measuring the distance between the mounting surface and the yardstick.
It can also be calculated, as shown below. Don’t forget to add for the rim’s lip and tire bulging past the rim (I use about .25″). Our 16×8″ 0mm offset front rims would have 4.25″ of backspacing. The positive 16mm rear rims (16mm/25.4 = 0.63″) have about 4.9″ of backspacing
|Rim Width / 2 = Centerline
Centerline + Offset + Rim Lip = Backspacing
8″ wide rims / 2 = 4″ Centerline
Measuring Maximum Backspacing
The 89 Camaro can fit rims with a maximum of 5″ front and 5.85″ rear backspacing. This can be measured, or found out the hard way through trial and error. To measure it, bolt a flat piece of metal or wood to your hub using 2 studs. Towards the end of the metal, drill a hole and screw a long bolt through it. This bolt should be 1 radius from the hubs centerline (If you will be putting on 26″ dia tires, the bolt should be 13″ from the hubs centerline). Spin the measuring tool around, and screw the bolt in untill it no longer touches anything. The length of the bolt is your max backspacing. If you have suspension pieces jutting in the wheelwell, try different bolt locations to make sure nothing is in the way. Do this with the wheels turned left and right as well.
Measuring Bolt Circles
All Camaros and Firebirds had 5 lug on a 4.75″ bolt circle. If you’re adapting to a different bolt circle, it can get tricky to measure it. Four and six lug bolt circles can be measured from the center of a stud to the center of the opposite stud. 5 lug bolt patterns have a bit of trigonometry involved. The online 5 lug bolt pattern calculator below will give you the bolt pattern based on a quick measurement.
5 Lug Bolt Circle Calculator
Adapters move the rim further out. They let you bolt on rims with too much backspacing. If you decide you want to bolt on a pair of Corvette ZR1 rims (17×9.5″, 50mm Offset) you would first need to calculate the backspacing.
9.5″ / 2 + 50mm / 25.4 Offset + 0.25″ Lip = 6.97″ Backspacing
Rim Backspacing – Max Backspacing = Adapter Thickness
6.97″ – 5″ = 1.97″ Front Adapter Necessary
6.97″ – 5.85″ = 1.12″ Rear Adapter Necessary
Ideally, you would want some clearance on the inside of the rim to compensate for suspension movement, etc. In this case, we recommend 2″ adapters for the front and rear. You can run the 2″ adapters in the rear, even though you only need an extra 1.12″ backspacing. This will move the wheel further out in the wheelwell, matching the front, and the wider track will help cornering as well. The only time to not match the front adapters is when you have wider wheels in the back (17×11 ZR1’s for example). In that case, the wheel will fit in the wheelwell if the 1.25″ adapter is used, but will protrude past the fender lip if you have a wider than necessary adapter.
For all fourth-gen rims – 2″ adapters front and rear should be used on a third-gen. Some folks prefer having a thinner adapter in the rear. This will fit fine, but the rim will be inside a bit compared to the front. The same thickness on the front and rear gives a more balanced look than using thinner adapters in the rear (with the same size rims f&r;)
For C5 and 17×11 ZR1 rims, please do the math!
Using a 4th Gen Rear
If you’ve changed your third-gen rear to a 4th gen rear, you do not need adapters. The 4th gen rear is approximately 2″ wider per side. This is the same as using a set of 2″ bolt on adapters
You have two choices when the rims need to be moved out – slip on spacers or bolt on adapters. Slip-on Spacers
These are the discs that slide over your studs. For small thicknesses (1/4″ or less) they are fine. For greater thicknesses, the wheel studs are seriously weakened. The reason is that the studs have a greater bending moment since the lugnuts are further away. Under braking or acceleration, the studs would be stressed more, and might break.
Billet adapters bolt onto the car just as a wheel would. The adapters have their own studs that the rim bolts onto. This design is just as strong as without adapters, since the stud forces haven’t changed. The adapters are made from a billet cylinder of 6061 T6 Aluminum. The billet is machined to fit over the hub, with stud holes drilled to fit. Studs are pressed through the adapter to which the rims are bolted onto. The adapter is then slipped over the car’s studs and bolted on. The rims finally bolt on as usual.
There are two ways of bolting the adapters to your hub. One uses a lugnut with a long shank and washers. The clearance needed for the holes ends up as a slight bit of slop when they are bolted on. This isn’t usually noticeable when driving. The other method is to use acorned lugnuts to bolt on the adapters, just like for the rims. The lugnuts center the adapter when they are tightened. This is a significantly more precise way to bolt on the adapters, and is how we make them.
Various Rim Dimensions
|Rim Name||Size||Offset||Backspacing||Suggested Thickness (3rd Gen)|
|Corvette GS ZR1 style||17×9.5||50mm||7″||2″|
|Corvette ZR1 style (Low Offset)||17×9.5||38mm||6.7″||2″|
|Corvette ZR1 style (Low Offset)||17×11||36.5mm||7.397″||1.25″ Rear|
|Corvette ZR1 style (High Offset)||17×9.5||56mm||7.4″||2.25″|
|Corvette ZR1 style (High Offset)||17×11||50mm||7.93″||2″ Rear|
|C5 Corvette 97-2000||17×8.5 front||56mm||6.8″||2″|
|C5 Corvette 97-2000||18×9.5 rear||63mm||7.6″||2″|
|C5 Corvette Z06||17×9.5 front||54mm||7.2″||2.25″|
|C5 Corvette Z06||18×10.5 rear||58mm||7.9″||2″ Rear|
|C6 Corvette||18×8.5 front (GM 9594351)||58mm||6.53″||2″ Front & Rear|
|C6 Corvette||19×10 rear (GM 9594353)||79mm||8.11″||2.25″ Rear|
|C6 Corvette Z06||18×10 front|
|C6 Corvette||19×12 rear|
|Camaro 82-92||16×8 front||0mm||4.35″|
|Camaro 89-92||16×8 rear||16mm||5″|
|Camaro F1 ZR1-style||17×9||50mm||6.8″||2″|
|ROH ZS||17×9.5||5 mm||5.472″|
|ROH ZS||17×8.5||-7 mm||4.5″|
Disclaimer – These dimensions have been gotten from many sources, and are not verified. Use at your own risk. If you see an incorrect number, please let us know. The suggested thickness may not match based on offset numbers. They are based on experience fitting those specific wheels to third-gen f-bodies, and reflect feedback from customers.