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Offset Question


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I'm in the market for 13x6 wheels, and was trying to do the math for the correct baseline offset specs.

So I know on a 7 inch wide wheel, +25mm offset is ideal.

 

Then that means on a 6 inch wide wheel, i'd divide that 1 inch difference in half, convert that half inch to mm, and subtract that from +25mm (7" wide ideal offset), right? I think my logic is sound here, but want to lean on the experts on this forum lol.

 

  • .5" = 12.7mm
  • 25mm - 12.7mm = 12.3mm, or round up for an ideal offset of +13mm on 6 inch wide wheels?

'72 Metallic Malaga 2002 with a '73 M10 | Dual Weber DCOE 40s | E21 5spd and LSD | Pertronix II

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53 minutes ago, 2002iii said:

I think you've got that wrong. The e30 bottle caps are 14x6 et35 and they fit nicely.

 

I believe you were supposed to add the 12.7mm. So 25+12.7=37.7

The 14X6 et35 e30 bottle caps sit a little too far in for 195 tires. Depending on the brand, there is a good chance they will slightly rub against the tie rod. If they don't rub , there is very little clearance. When I ran them I mounted 185/65 x14 tires for more clearance. An offset of about 25-30 is better for a 6" wheel. I couldn't imagine fitting 7" wheels and tucking the tires under the bodywork. 6" is pushing it.

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Also to add:

  • My current wheel setup is a set of E30 325ix basketweaves, 15x7 et41 with 15mm spacer, on 195/50 tires.
  • Car is lowered 2 inches all around, but no camber plates up front...yet
  • I love the way the tire/rim combo is flush with the fenders.
  • My fenders are rolled on all four corners.

I know the Borrani steelies are 13x6 or 6.5, do you guys know what offset those are? I remember them being in the single digits...

Edited by 72MetallicMalaga

'72 Metallic Malaga 2002 with a '73 M10 | Dual Weber DCOE 40s | E21 5spd and LSD | Pertronix II

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It all depends on what you’re trying to accomplish.

 

If you want the outside lip to be the same, 6” ET12 is the answer. That’s equivalent to the 7”ET25 with 1” removed from the inside lip.  This is the solution if you want to fill the wheel well and are running a relatively standard tire width for 6” wheels, say a 175+/-
 

If you want the center point to remain the same, 6”ET25 is the answer. This will be equivalent to the 7” wheel with 1/2” removed from the outside and inside lip. This is the solution if you want to run a wider tire, say 195 and up. 
 

If you want the inside lip to remain the same, 6”ET37 is the answer. This will be equivalent to the 7” wheel with the 1” removed from the outside lip. This is the solution if you want the car to have a comically narrow track width and look like one of those trucks that drives on the train tracks. 
 

Realistically, you’d likely be looking for an offset in the 12-25mm range. Depends on what you plan to do about tires. 
 

*edited for math typo

Edited by Lucky 7
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2 hours ago, Lucky 7 said:

It all depends on what you’re trying to accomplish.

...

Realistically, you’d likely be looking for an offset in the 12-25mm range. Depends on what you plan to do about tires.

Lucky 7 explanation above is very much true but focused on outlook. Offset has great effect to the car handling too. When going towards lower values steering effort is increased and the steering wheel hits your hands at each pothole or a step from driveway. I have experience running ET22 (15x7 with turbo flares) and I would not want to try ET12. Unless you want to focus on looks and not worry about driveability.

Also the space inside fenders vary from car to car. I have aftermarket front fenders that are a bit narrow and my current setup 13x6" ET30 with 185/60 tires just barely clear the fenders without rubbing.

Edited by Tommy

Racing is Life - everything before and after is just waiting!

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