Just to start off: I didn't want to write a book here, but it seems to have turned out that way.

I have been wanting to get some new tires and wheels and, of course I want to go as wide as possible (within reason), but I don't want the tire sticking out past the fenders. To be specific, I don't want the bulge of the tire passing beyond the plane of the fender.

So I took some measurements to see how much clearance I have. This turned out to be about 0.9" outboard and about 1.5" inboard. So, being the **** retentive geek that I am I put together a spreadsheet to help me out. I think all my calculations are correct except for the correction factor for determining offset. Because of the different points of reference for measuring wheel width and wheel backspacing, a correction factor is needed (basically this is the width of the wheel lip). Now, I don't think this number is standard and more than likely it varies from wheel company to wheel company. So, I used what worked with my stock wheel: It is cast as 16x7x31 and has a backspacing of 5", so I used .2795 as my correction factor.

I think that this spreadsheet could be useful to a number of people that are contemplating new wheels/tires.

From everything that I've seen no one has taken into consideration the section width of the current vs. new tires and this is what I am addressing. This is where your clearance problems arise, IMHO. It looks like I might go with 295/45 series instead of the 40 series because they have an additional 300 pound load rating and the larger diameter difference is about the same as the other are smaller.

Do you guys think this is a useful tool?

http://users.adelphia.net/~snakeoiler/TireSizing.xls

 

If anybody cares, I have updated the spreadsheet at the link above. Based on what I have found (in many places), my correction factor (that I speak of above) should be .5" - it obviously is a standard.

This actually means that the stock aluminum wheels that are cast with the 16x7x31 size actually have a backspacing of 5.25" (I confirmed this the other night by measuring my wheel) and not the 5" that has been spoken of elsewhere, that I have found.

 

I'm running 295/45-18 up front with no issues.
18x9.5 rim with a 6" backspace

2wd Hotchkis 2" drop coils.

I used a wheel tool to figure it out.

I Couldn't get your spread sheet to work but it sounds like a neat idea. Should help some guys out. I'll have to try it again.

 

I was running 295/45-18's on the back, no rubbing. They tucked very nice.

 

I'm planning on 295/45/20, front and rear. I have the tire mount mate tool, so I'll know for sure. Why someone never came out with this tool 20 years ago - I'll never know.

If I have to I will go with 265/50/20 up front only. I'm at stock height right now and I may do a 2" drop in the rear. I thought about a 2/4 drop, but I think then I definitely would have to go with the 265's