bull30

I finally got tired of the run around from the various tire places about offset... So, I created a spreadsheet to help fellow Rennlisters... All you do is fill in the light yellow cells for your stock wheel then fill in the light yellow cells for the proposed wheel... The increase or decrease in width is automatically calculated... Then you can measure your Porsche to see whether or not the increased width will fit... Obviously there may be other factors but this will get you close enough to make a decision...

Page 2 of the spreadsheet includes a tire size calculator... I'll be adding a speed differential column to this spreadsheet shortly...

IamSMC

Thx for taking the time and sharing

Bill Verburg

To make it truly useful you need to be able to input width, o/s and either flange thickness, frontspace or backspace.

As a last resort you can estimate flange width it is usually between .375" and .5"

bull30

That is a minimal number since it would only be the difference between the flange on your existing wheel and the new wheel... The main number needed is the increase or decrease in overall width... You still have to crawl under the car and take some measurements to see if the new wheel will work...

I called Wheel Enhancement the other day and gave them the info on my car and asked what wheels would fit and what offsets I should get... You would I was asking for info that violated national security... I got the same run around from Tire Rack...

At least now I have something that will get me close enough to make an informed decision...

Greg H.

What is this flange thickness? If I get a thicker flange, do I end up with less threads engaged in my lug nuts? And is that a problem?

Greg H.

bull30

We're talking outer flange of the wheel where the tire bead sits up against... The thickness can very from wheel to wheel but that difference is minimal... The calculator will get you close... It's most important to crawl under your car and do some measurements... Use a plumb bob from the inner edge of the top of the wheel well then measure from the string to the outer most edge of the wheel / tire... Then measure to see how much room you have from the inner fender well and suspension parts... Sometimes you have more room there and you can gain more wheel width by increasing offset... For example a wheel with an offset of 67mm will set inward more than a wheel with a 55mm offset... If you have the inward room you may be able to use a wider wheel with more offset without worry of rubbing the outer fender well...

The problem is that most tire places are not going to take the time to go over all this with you... They will sell you whatever they know SAFELY fits without liability to them... Most of us want to fill up our wheel wells... I know that's my goal...

Hope I helped...

Bill Verburg

When stuffing big rubber and tires a silly little mm can make a big difference, I always include it in my calcs.

Antonov

http://www.1010tire.com/WheelOffsetCalculator.asp here is another easy to use offset calculator

David in LA

IMHO wheel offset calculators are just the first step since as others have pointed out it doesn't take into account different flange thickness.

But because the same size tire from different makes can vary in tread width by up to 10-15mm (in my experience) and can also vary in overall width by up to 20mm (again in my experience), aside from an actual fitting if you are looking for an aggressive fit you need to go by what others have been able to fit on their cars. And all the while still keeping in mind that there are slight variances from car to car and even from the right and left sides of the same car which may affect your fitting. Also keep in mind that as the suspension moves up and down that will affect clearance (both positive and negatively)

IIRC Bill had put together a list of wheel sizes, offsets and tire size/brand combos that have proven to work and I would say he is one of the best resources as far as proven fits. What Bull has put together is very useful for looking at new fitments.

993_Pilot

I did an Excell spreadsheet a while back too. It has it's limitations, but I think that it works pretty well. I don't mean to Hijack or compete with Bull30, but here it is ...