rim fitment question 17x9.5 275/40/17

1968_stang · Mar 26, 2009

Hello. This is my first post here but have been a long time reader. I've been doing my research on new rims and tires for my 68 coupe. Currently everything is stock but I am planning a 9" axle swap soon with the cobra disc brake conversion. My current rims will not accomidate these brakes so I was looking for 17" wheels.

Now for my question. I am looking at the MB wheels Old School rims from discount tire. I want to do 235/45/17 in front and 275/40/17 in back. From what I've read this should work with the correct backspacing.

The size rims I was looking at was 17x8 7mm offset for the front, and 17x9.5 6mm offset in the rear.

What is everyones opinion on this fitting? I am also looking at getting TCP G-Bar kit or RCs coilover kit in the future.

1968_stang · Mar 26, 2009

Forgot to add, will I run into any problems with those suspension upgrades and these wheels and tires?

JC6715 · Mar 26, 2009

I am running a G-bar and 315/35/17's in the back of my '67. The wheels are 11" wide with 6.5" of backspacing. It's going to require a bit of clearancing on the inner wheel house, but not what I would call a mini-tub.

That being said, yes your 275's will fit once you get the right backspacing. If you go the G-bar route it will only help with the clearance and spacing as the lower control arm is slightly more narrow that the leaf spring.

SNAKEPILOT · Mar 26, 2009

The wheel widths and tires you specify will work but the proper backspacing is critical.

Most of us on this site are used to working with backspace, not offset. If I did the math correctly, your front wheels will have a 4.25 backspace and the rear about 5.00. That isn't going to work front or rear.

For an 8 inch wheel up front, you will need at least 4.5 and I think 4.75 works better (that is what I have). In the rear, for a 9.5 rim, you will want between 5.5 and 6.0. With a 5.5 you will likely need to roll the fenders. With a 6.0 you should massage the bump in the forward part of the wheel well and trim the bumpstop. 5.75 would be ideal but that is not a common backspace.

The brakes you use may affect your backspace requirements. I am not familiar with the Cobra rear kit (or was that just for the front?). Make sure it does not increase your track or compensate accordingly.

Grabber70Mach · Mar 26, 2009

SNAKEPILOT said:
The wheel widths and tires you specify will work but the proper backspacing is critical.

The brakes you use may affect your backspace requirements. I am not familiar with the Cobra rear kit (or was that just for the front?). Make sure it does not increase your track or compensate accordingly.

Agree with Snakepilot, you have a lot of variables with your plan. Depending on the kit you get they adjust your track width, also whatever rims you get now might not work with the G-bar or RCs kit.

1968_stang · Mar 26, 2009

SNAKEPILOT said:
Most of us on this site are used to working with backspace, not offset. If I did the math correctly, your front wheels will have a 4.25 backspace and the rear about 5.00. That isn't going to work front or rear.

Thanks for the reply. My math was the same. I started getting confused when I was using this site:

Wheel / tire size calculator / comparer - WHEELSMASTER

This has a different measurement for backspacing. Looks like the backspacing is 5.5"

I also found this chart (bottom of the page) telling me the same:

https://www.rsracing.com/tech-wheel.html

Is there a different way to calculate backspacing when offset is the only thing given?

1968_stang · Mar 26, 2009

I did some more research on backspace vs offset. Here's the site I found that explains it, makes sense

Mickey Thompson Performance Tires & Wheels

So the only thing that could change is the conversion from drum brakes to disc? Does anyone know how much this changes things? The 9" axle I'm using is the standard width for the mustang, so the axles should be in the same position no matter what? just the rotors changing things?

xplane · Mar 26, 2009

Hi,

I'm also new here, and have have several projects i'm working on.

I have been test fitting a set of 18/9"F & 18x10"R BBS RK's. This is on a 69 Mach I so there is a little more room. With fenders rolled on both ends, I plan to run 245/35ZR18 fronts and 285/30ZR18 rears. This will be very tight, with less than a half inch clearance per side.

I also plan to run the G-bar setup and would like input from any one with personal experience using this setup.

Would love to see pictures of your car 1968_stang!

I also have a 68 GT coupe.

1968_stang · Mar 26, 2009

Here's the picture's. Nothing to fancy, but she's mine!

xplane · Mar 26, 2009

Looks nice, which style MB wheel are you going with?

They make some nice looking wheels. I hadn't looked into these before.

Mike

1968_stang · Mar 26, 2009

Thanks!

I was looking at the Old School's. From what I've been reading they just came out this month so there isn't much usable information on the internet about them. The Discount Tire site seems to be down right now so I can't call and verify the backspacing. I did call one of their stores in CA since they are all closed on the East Coast right now. The guy wasn't any help. he said they don't have that information until the wheels come in. he did quote me a different price then what was on the internet, by $20 a rim though

xplane · Mar 26, 2009

I was wondering why I couldn't find those on the MB website. Where did you see them?

Mike

1968_stang · Mar 26, 2009

Discounttiredirect.com is the only distributor of the wheel. Here's a picture in gunmetal, I'm looking at the chrome version, but their website is down right now for some reason.

xplane · Mar 26, 2009

Those look nice, remind me of Coy's C-55's. I have Coy's C-5's on my '61 Buick Invicta.

Coy's Custom Wheel & Tire

Mike

1968_stang · Mar 26, 2009

Very similar actually. The price of the MB wheels are much cheaper though. $188 i believe for the 17x9.5. Free shipping too

xplane · Mar 26, 2009

Looking forward to seeing these on your car, should really set it off.

I am kinda on the fence on my 68. I'm not sure if I should sell it or turn it into a track day road race car. Willow Springs Raceway is right down the street, hard not to join in the fun!

Mike

1968_stang · Mar 26, 2009

It almost seems like they are the exact same rim, the Coys might be easier to get information on than these new wheels.

Grabber70Mach · Mar 26, 2009

1968_stang said:
I did some more research on backspace vs offset. Here's the site I found that explains it, makes sense

Mickey Thompson Performance Tires & Wheels

So the only thing that could change is the conversion from drum brakes to disc? Does anyone know how much this changes things? The 9" axle I'm using is the standard width for the mustang, so the axles should be in the same position no matter what? just the rotors changing things?

It depends on the kit you use. For instant the Vintage Venom kit I am using to run discs front and rear increases the front 1" and the rear 7/16", this doesn't include the thickness of the rotor hats. Here's some pics of mine with the Cobra brakes and 17's.

1968_stang · Mar 26, 2009

I was looking at the vintage venom kit, did you use the SN95 Spacers with the kit? I don't understand if the caliper bracket bolts to the axle housing why that would affect the spacing.

Grabber70Mach · Mar 26, 2009

Yes I used there complete kit. The bracket bolts to their adapter which bolts to the axle housing. The hubcentric spacer fits between the rear axle and the rotor.

rim fitment question 17x9.5 275/40/17

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