I recently went through the process of calculating wheel offsets and fitting tires, and wanted to share some of that knowledge to inform future buyers for their next set of wheels. This is sort of an update to the FAQ for Wheels and tires.

Camaro Stock Wheels:
16"x8" +55
Camaro Stock Tires:
245/50/16

Taking an approximate measurement from the fender lip, I measured about 1.5" between the rim lip and the outside of the front fender. For simplicity, I am going to just apply this same calculation to the rear, although you can probably get away with another 5mm or so in the back with a rolled fender.

Offset is the spacing between the hub mounting face and the center line of the wheel.

So to calculate the spacing from the hub to the outer lip, here is what I did:

Convert 8" width to millimeters: 8 in x 25.4 mm/in = 203.2 mm
Divide that width by 2 for the front half of the wheel = 101.6 mm
Subtract the offset to get the front spacing = 46.6 mm
Converted back to inches: 46.6 mm / 25.4 mm/in = 1.83" from the hub face

After mounting my new wheels with spacers, I found that wheel lip was just outside of the fender at 9.5" width, +35 offset with a 5/16" spacer to clear the brakes.

My wheels: ESR SR04 18"x9.5" +35 (3.37" from lugs, +1.53" from stock)

So to get to the fender with a 9.5" wheel would take about a +30 offset, which would be 3.56" from the hub, or about +1.73" from the stock wheel. Now this calculation is TO THE FENDER, so if you run a setup like this, you will need a stretched tire if lowered.

Using these calculations, here are the lowest offsets you can run on stock fenders:
8": +11 mm
8.5": +18 mm
9": +24 mm
9.5": +30 mm
10": +37 mm
10.5": +43 mm
11": +49 mm
11.5": +55 mm
12": +62 mm

Now, to determine which tires to run, I guesstimate based on wheel width. If you convert, say, a 10" wide wheel to millimeters, the rim width becomes 254 mm. This measurement is typically between the inner beads of the wheel lip. If you are right on the fender lip, you will want the tread to be laterally on the same plane as the rim lip. So if you want to run a stretched tire without being Hellaflush and having your rim lip outside your tire lip, here is the rule I go by. Since the width of the rim is measured from the inner lip, you typically want the tire to be just above the rim width, so on a 10" wide rim, I would go no lower than a 255 tire, although a 265 tire would fit better (but is a much harder size to find).

The final calculation I made was tire diameter.

Tire Diameter = (width x aspect / 2540) x 2 + wheel diameter

The stock tire size is 245/50/16
(245 x 50 / 2540) x 2 + 16" = 25.64" Diameter

You will want your new tires to be approximately the same size as your stock tires to allow your speedometer to read correctly, however there are site that will calculate the size difference for you, but I prefer to see the actual difference in diameter from calculations.

Using these calculations, for an 18" rim, the closest sizes to stock are:
245/40/18 = 25.72"
275/35/18 = 25.58"

Hopefully this helps some people with wheel fitment in the future, please let me know if I can answer any questions.

 

Thanks for replying on your other thread. So your saying

"According to my calculations, your front wheels will stick out of the fender about 8mm (~5/16"), your rear wheels will stick out of the fender about 21mm (~13/16"). The rears can be rolled and pulled to fit. The front are going to stick out a good bit, so you will either need an even more stretched tire than I used, or you will need to get creative with the fiberglass fenders."

I have roh zs wheels that are 17x9.5 all around with a +35 offset. They almost seem flush in the front and for the rear they seem quite far away from the fenders.
I want to get esr 18x9.5 +22 for the fronts and 18x10.5 +22 for the rear. From the calculations i did it seems that in the front my wheels are going stick out 0.512 inches(13mm) more and in the rear they are going to stick out about 1.012 inches (25.7mm).

With +35 offset and a spacer you should be very flush. With a spacer you offset pretty much becomes +27. If your really flush with +27 then +22 should be out the fender. For the rear how far are you away from being flush? For all of this im still at stock ride height and planning a 1.25 inch drop in the future. Thanks!!!!!

 

Yes, the +22 ESRs will stick out 13mm and 26mm more than your current 9.5" wheels with +35 offset.

Let me rephrase the part about the rear wheels. Something like a +35 will put you flush with the inner fender lip without rolling the inside of the rear fender. You can probably make up another 5-10mm in offset by just rolling that inner lip. Not a requirement for stock height, but once lowered, you definitely need to roll the rear fenders.

With +35 offset and a custom 7mm spacer, that brought me to +28 offset, which did actually have me poking out of the fender about 2mm. Which is why I had to run slightly stretched 245 tire instead of a 255 or 275, because I had to actually run a tire slightly smaller than the 10" width of the rim outer lip (245mm = 9.64").

I don't see how you could possibly make +22 work out of the front fender. It will work on the rears with fender modifications, but I highly recommend doing no lower than +35 offset in the front, especially since I think you will need a spacer due to the spoke design. I believe on these wheels they shift the barrel for a deeper dish instead of spacing the face further from the hub.

I cannot take any measurements as I sold the car earlier this year, but I can advise you from the measurements I did take.

 

The only reason i thought about doing 18x9.5 with +22 offset in the front is because i do not like the idea of spacers. I would assume that thw +22 offset will help me clear the calipers. Ive seen one guy do it on instagram but he is really tight tothe fender and is car is slammed. I was thinking about putting a 255 35 18 in the front but it seems like i wont be able to do that. On stock ride height do you think ill rub or hit the fenders with +22 offset and 255 seeies tires. the only reason i want new wheels is because i want a wider size tire in the rear and 9.5 on mycurrent wheels really limits me. So having 10.5 +22 will easily let me go to a 305. I want give my car a wide stance and meaty look. Thanks forthe help, i really appreciate it!

 

Well like I said earlier, double check to see if the lower offset is achieved by the barrel or the hub mounting. I had to space the hub on mine by 7mm to clear the brakes. +22offset and 255 tires will definitely rub. Even a 245 probably will. I'd say do the +22 10.5's out back, but the 9.5 +35 out front with a 245/40 tire. Gives you that meaty look from the back without sacrificing too much driveability.

 

By wheel barrel does that mean back spacing? Im going to give esr a call right now.

 

So i called esr. They recommend that i should go with +35 and run spacers. The guy on instagram is running +22 and 9.5s. I guess the calipers in the lt1 z28 were smaller than my 2002 ss. For the fronts im ordering +35 but for the rears i think a +22 offset just might be too aggressive. They say 18×10.5 with a +38 offset is fully flush. 38-22 is 16mm or 0.63 inches out of my fender. I really want to lower my car 1.25 inches with out having a rubbing issue. Thanks once again

 

I have a 2002 WS6 and I’m wondering if these BMW rims will work? My car is lowered with Stranos and Konys. I don’t want to rolling fenders or anything. I know this is a very old thread, but I’ll take a chance and see if any one responds. The wheel and size are as follows 18” Apex Arc 8 Rims

Rim: 18x9.5

Offset: 35

Thanks for your help if you are out there. cheers

 

And this didn’t help?

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