Wheel Size / Clearance Issues?

[CV355]

I currently have 275/60R15s in the back and 28x6-15s in the front on my '69 Mach 1 CJ.  There is about 1/2" of clearance to the leaf springs.  Based on my measurements, the rear wheel looks like 6.75" backspacing, 3.25" dish, .75" center section (10.75" wheel?).  The fenders are rolled quite nicely.  Slight rubbing on the fronts on a hard turn. No rubbing on the rear tires.

I'm looking at putting American Racing VN427s on the car- either 15" or 17".

1)  My 428 has custom longtube equal-length headers.  The collectors are quite low as-is.  I'm sure some of you guys are running similar- any ground clearance issues with your current tire/ride height?  I'm afraid if I go with anything smaller than a 28" tire, I'll tear the exhaust off on a speed bump.  Did that 10 years ago on a GT crossing railroad tracks.  Bad day. 

2)  Most recommended tire sizes for the '69 are at 25.7" diameter, which is a 1.15" drop from the current height.  Staying at the same height in the rear is no problem- love the look and I can re-use the MT ET Street tires.  Thinking a 10" wheel- not sure on backspacing though due to the existing clearance issues.  I trust my measurements but I'd like to hear from the rest of you guys. 

3)  What backspacing would I have to run in the front to keep a 28" tire but with more contact patch?  Maybe a 7" width?  Do I really want to keep a 28" front, or should I consider something else?

Again, not the most technical of questions, but I'm having terrible luck with vintage car questions everywhere else.  I'll get into the FE technical questions at some point, I promise!

Thanks for the help so far!

[Jb427]

Is there car low? we run 245/50 15's on a 8.5" rim front and 265/50 15 back on a 10" rim i will find out the offset for rims on 69 mustang. Those are on a mustang that is very low a bit too low since the scatter shield hits speed humps. if i remember right i think had a 10 mm spacer on the front wheels to push them out.  The 245/50 15's may look odd if your car is not lower then std 

[CV355]

Is there car low? we run 245/50 15's on a 8.5" rim front and 265/50 15 back on a 10" rim i will find out the offset for rims on 69 mustang. Those are on a mustang that is very low a bit too low since the scatter shield hits speed humps. if i remember right i think had a 10 mm spacer on the front wheels to push them out.  The 245/50 15's may look odd if your car is not lower then std

28-6-15 is 152/108-15, just for comparison sake.  I'd be happy with a 245 width tire in the front- would definitely help with handling (not that this car is getting thrashed, but drag racing front-runners can be a pain with manual steering on the road).  A 24.6" tire would drop me 1.7" though, which would be dangerously low.  I think I have 3.5" of clearance from ground to the collectors now.  I'll measure when I get home.

Here's a pic of the car.  It doesn't look low, but that's probably because of the 28" tires.

[Jb427]

Ya the collectors are always a prob some of the best money I spent on my fairlane was on headers from "ford powertrain" its now one of the only cars i've had that dose not scrape.
here is a few pic's of 245/50 on 15x8.5" rims and 265/50 15x10"on the 2 cars first one is a 351c xy gt no longer owned historic race car and 66 fiarlane old photo hood is now painted. You can see the ride hight and stance but more of a cruiser/circuit racing look over a drag racing stance.

[CV355]

Ya the collectors are always a prob some of the best money I spent on my fairlane was on headers from "ford powertrain" its now one of the only cars i've had that dose not scrape.
here is a few pic's of 245/50 on 15x8.5" rims and 265/50 15x10"on the 2 cars first one is a 351c xy gt no longer owned historic race car and 66 fiarlane old photo hood is now painted. You can see the ride hight and stance but more of a cruiser/circuit racing look over a drag racing stance.

Wow those are way lower than mine.  I guess I could stand to lower a little.  If I have clearance issues I may bite the bullet and put some mid-length headers on.  Everything else should be ok.  I don't think the car was lowered.

[Falcon67]

On the 70 Mustang I use 15x8 rims with 4.5 BS and a 28" drag slick or 275-60 drag tire.  Lots of clearance to the leaf spring and no fender issues.

[CV355]

On the 70 Mustang I use 15x8 rims with 4.5 BS and a 28" drag slick or 275-60 drag tire.  Lots of clearance to the leaf spring and no fender issues.

Boom- exactly what I needed.  How's the contact patch on the 275?  Must be nice and fat on an 8" wheel.  Do you have pics?

[Falcon67]

I can get some, will try to do so tonight. - EDIT found some LOL

[CV355]

I can get some, will try to do so tonight. - EDIT found some LOL

Awesome!  That's what I'm going with then.  Lets me keep the existing tires too. 

[afret]

28" tall M/T Sportsman tires on 15x5.5 wheel on the front,  275/60-15 on 15x10 with 6.125" BS on rear.  About 3/8" clearance to leaf spring.





27" tall skinnys on 15x3.5 wheel,  28/9.0-15 Pro Bracket Radial on 15x10 with 6.5" BS.  Similar clearance to the leaf spring as above since the tire sidewall width is skinnier than the 275.

[TomP]

On Chris' pictures that wheel needs more backspacing by 3/4" or so. The bump stop can be trimmed back and the leaf spring is the next obstruction. I have run tires within a 1/8" of the springs without much issue but that outer wheel lip will tear up the sidewall quick.

[Heo]

As i remember it the front inner lower corner of the wheelhouse
is where it hits first...or maby thats just on earlyer mustangs

[machoneman]

I'll only add my '70 Mach 1 experience with street/slalom use. Getting high performance (read: high speed rated) 15" tires today is very difficult. Have 16"s on it now as in '94 that was the way to go. Today, even 17"s have a somewhat limited selection of top-speed-rated tires. Have looked at 18"s and quite a few selections are available and 18's/19's look weird.

Again, not the most technical of questions, but I'm having terrible luck with vintage car questions everywhere else.  I'll get into the FE technical questions at some point, I promise!

Thanks for the help so far!
[/quote]

[CV355]

This is going to sound ignorant, but we are talking about negative backspacing, correct? 

Some of the websites I'm looking at do not specify negative/positive backspacing and the wheels I'm looking at (VN427) are custom, no returns

[jayb]

I've never heard of negative or positive backspacing, the measurement I'm familiar with is the distance from the wheel mounting surface to the inside of the wheel.  Put the wheel upside down on the ground, lay a straightedge across the back of the wheel, and measure down from the straightedge to the wheel flange mounting surface.  That is the backspacing dimension that is generally published.