front backspacing

[g49bridges]

Hey guys, sorry if this has been discussed already...didn't find it if it has. Trying to see if my front wheels will fit before I buy them. 74 duster, lbp w/discs. Currently have factory 14' rallys with 245/60r14s. Wanting to go to a 225/60r15. The wheels Im wanting are 15x7 and only available with 3.75 backspacing. Just wondering if this will fit or if I need to look at different wheels with more backspacing?

 

[72bluNblu]

To run a 225/60/15 up front you'll need a 15x7 with 4.25" of backspace. Even that's pretty tight, some cars need some trimming at the lower front corner of the wheel opening. When I had cop rims and 225/60/15's on my Duster I had to max out the lower fender corner to bumper braces to make it work.

 

[g49bridges]

I've got 245/60r14s on now, which seems to be the same diameter, but 1" wider than a 225/60r15. So just curious if the backspacing would still be off with the narrower tires. Also, I can get the wheels I'm wanting in a 15x6, which should work with the 225/60r15s, but they too have the same 3.75 backspacing. So would the 6" rims work? I am by no means a wheels and tires guy, so excuse the learning curve...lol

 

[AJ/FormS]

The math puts the diameters of those two nearly identical, at 25.6inches.
-The 14"rallys were,IIRC, a 5.5 wheel with a 6.5" max outer width. IDK the backspace on those, but might assume about 3.25 for a Zero offset.In which case the outer edge of the rim would be 3.25 outboard of the mounting flange.
-Your new wheels are 15 x 7, which is 8" max outer width. You specify a 3.75bs. This puts the outer edge at 4.25 outboard of the mounting flange.
- Meaning........The new wheels will push the tread 1 inch further into the fender, at the straight-ahead position, and will push the treads out 1 inch closer to the front lower corners of the fenders, when cornering.
-A 6 inch wheel with 3.75 bs, being 7'' max outer width, puts the outer edge at 3.25 outboard of the mounting flange, same as my assumed rally wheel.

 

[72bluNblu]

What width rim are the 245's on? What's the backspace?

If they're factory rallye's they're probably a 6" wide rim at most? Meaning, the 245's are really squeezed on there, that rim is a full inch narrower than recommended for that tire. That significantly changes what the tire profile looks like.

That also changes the location of the outer edge of the tire. Remember, if two rims of different widths have the same backspace, they don't have the outer edge of the rim in the same place, the centerline of the rim has moved. The easiest way to look at that is the rims offset. A 15x7 with a 3.75 backspace is a -6mm offset. A 15x6 with a 3.75" backspace is a +6 offset (like most of the factory rims were, BTW). That difference means that the outside edge of the rim moves by a 1/2" between those options, even though the backspace is the same. Offset keeps track of the centerline of the rim even when the width changes, backspace does not.

With a 15x6 that has 3.75" of backspace a 225/60/15 should fit the car fine, and the 15x6 is still in the recommended range for the 225/60/15's.

 

[g49bridges]

Thanks for the info! I've never really been up on how to figure wheels and tires, but that makes sense the way you guys explained. And yes, the 6" rim, according to summit, is within the recommended size for the 225/60/15s. It is the minimum size recommended, but should work. Now I can order with some peace of mind!