Would someone explain wheel off-set numbers? I've never had to decide what off-set to chose when buying wheels. I understand the off-set on my 2012 GT500 stock wheels are, - Rears 20x9.5 (+50) - Front 19x9.5 (+44). The BMC wheels I'm considering have the following off-sets, Front +25 Bolt Pattern 5*114.3, Rear +37 Bolt Pattern 5*114.3.

If I install a set of BMC's on my car will the wheels stick out further or be recessed in further?

Catmonkey will chime in shortly and school us on this topic. I'm clueless myself.

 

Wheel offset is the distance from the mounting surface to the centerline of the wheel, measured in millimeters. Positive offset, mounting surface is closer to the outside of the wheel, negative offset, it's closer to the inside of the wheel. Backspace is the distance from the mounting surface to the inner edge of the rim, measured in inches.


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That's a good graphic. I'd seen it or one like it somewhere before.

 

Lol yeah. Whiz bang boom!

 

If I understand the information correctly the BMC outside of the rear wheel will be tucked up into the fender well approximately 1/2" and the front approx. 3/4" more than stock. Did I figure it right?

Stock - Front 19x9.5 (+44)
BMC - Front 20x9 (+25)

Stock - Rears 20x9.5 (+50)
BMC - Rears 20x10 (+37)

 

Both will stick out farther than stock.

Front wheels - http://www.willtheyfit.com/index.ph...offset=44&width2=255&aspect2=45&wheel_size=20&wheel_width=9&offset2=25#isPage=1

Rear wheels - http://www.willtheyfit.com/index.ph...ffset=50&width2=255&aspect2=45&wheel_size=20&wheel_width=10&offset2=37#isPage=1


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I use this website a lot to figure out changes RIMSnTIRES.com. If you put my specs in 325/30-20, 20x11 with a 61mm offset, your combo should be fine as long as it doesn't go outside my dimensions. You're not likely to exceed my inside limit, but if you did, you could expect some rubbing. If your selection exceeds my outer measurement, you run a good chance of contacting the outter edge of the fender with your tire on a hard hit when the suspension bottoms. More positive offset moves the tire toward the center of the car. Less positive offset moves the tire outside of the wheel well. Look at how much space you have with your existing set-up and measure the distance to the fender lip. That should give you a good base line if you want to use your current wheel/tire combination. A 37mm vs a 50mm offset should move a similar sized tire 13mm (.5") closer to the outer fender. If you go with a wider tire, half the difference in width will add to that measurement. There is a reason Shelby recommended a 275/35-20 with the Alcoa wheels. Widening the wheels makes for a better offset by adding half the increase in width to the offset spec.

Shorter tires will be more forgiving if they do stick out the fenders. 285/35-20 and 325/30-20 share the same overall diameter at 27.7". The latter is significantly wider.

 

Now I am confused. Of course it doesn't take much to do that. The BMC wheels I'm considering have the following off-sets, Rear (+37), Front (+25).

When I as the offset question to the vendor they told me "This wheel kit is staggered and has the correct offset for my car."

 

The BMC wheels I'm considering have the following off-sets, Rear (+37), Front (+25).

When I as the offset question to the vendor they told me "This wheel kit is staggered and has the correct offset for my car."

That is a loaded question. See my example above. There's a lot more room for a larger tire with less positive offset. A 285/35-20 or wider can contact the outer fender if the suspension bottoms with a 20x10 and 37mm offset.

If you want wheels with the right offset, look here.

 

Here's an example of what I proposed above. My tire spec, which I think is probably as wide as you can go without rubbing issues, is on the left. On the right is the Shelby Alcoa 20x10 wheel with Shelby's recommended 275/35-20 tire size. Start increasing the tire size and you see that the tire exceeds my outer measurement. My tire just tucks on the outer lip. I may experiment with a 6mm spacer as opposed to the 8mm I'm running. I think my occasional inside rubbing will decrease with the shorter rear tire (27.7" vs. 28.7")

 

Actually what I would like to accomplish is having the same size wheels and tires all of the way around without any rubbing or steering and suspension issues. Is that possible?

 

Yes, it is. But 285 is probably the limit on the front unless you went with coilovers. If you look at my sig pic, I have a 285/40-19 on the front of my car. Now it is slightly taller than a 285-35/20, but not by much.

 

Like catmonkey, since it is one of the more common questions about wheels. I wish everyone would give a thorough description of your wheels in your signature. It sure would be helpful.

 

Here are the original Super Snake Shelby Performance Parts Alcoa offsets and the BMC offsets, as provided by the manufacturers for comparison.......

SPP Alcoa offsets

Shelby Forged 20"x9" (front) 24mm offset
Shelby Forged 20"x10" (rear) 36.7 mm offset


BMC repro Alcoa offsets

OFFSET FRONT WHEELS +25 Bolt Pattern 5*114.3
OFFSET REAR WHEELS +37 Bolt Pattern 5*114.3

With the axle centered under the car, the lip of the original Alcoa rim is very close to the edge of the fender lip, which makes the tire sidewall bulge "even with" or "past" the fender lip depending upon the tire choice, some enthusiasts like/prefer a more "tucked" look and don't like tires sticking out past the fender lip.

As shown in the comparison above, the BMC's stick out even more (+25mm vs. +24mm on the rear and +37mm vs. +36.7 on the front),<<this is not alot more, but if the Alcoa's were on the ragged edge of being the wrong offset for the S197 Mustang according to some, the BMC's are not a better fit. I had a picture of a BMC install looking down the side of a S197 Mustang, with the axle correctly centered the BMC wheels did look like they stuck out more than my Alcoa's.






R

 

Here are the original Super Snake Shelby Performance Parts Alcoa offsets and the BMC offsets, as provided by the manufacturers for comparison.......

SPP Alcoa offsets

Shelby Forged 20"x9" (front) 24mm offset
Shelby Forged 20"x10" (rear) 36.7 mm offset


BMC repro Alcoa offsets

OFFSET FRONT WHEELS +25 Bolt Pattern 5*114.3
OFFSET REAR WHEELS +37 Bolt Pattern 5*114.3

With the axle centered under the car, the lip of the original Alcoa rim is very close to the edge of the fender lip, which makes the tire sidewall bulge "even with" or "past" the fender lip depending upon the tire choice, some enthusiasts like/prefer a more "tucked" look and don't like tires sticking out past the fender lip.

As shown in the comparison above, the BMC's stick out even more (+25mm vs. +24mm on the rear and +37mm vs. +36.7 on the front),<<this is not alot more, but if the Alcoa's were on the ragged edge of being the wrong offset for the S197 Mustang according to some, the BMC's are not a better fit. I had a picture of a BMC install looking down the side of a S197 Mustang, with the axle correctly centered the BMC wheels did look like they stuck out more than my Alcoa's.


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Catmonkey, I certainly appreciate your taking the time to explain everything. With that information I do not want the tires or wheels extending outward any more than the stock configuration. This is a little disappointing. I guess it's back to the drawing board.
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I'm glad you understand it all. Not everyone does. While certainly not a cheap alternative, True Forged makes a similar styled wheel and they can make it with custom offsets. Here's a thread on that wheel design. Their website leaves a lot to be desired, but phone call to Steve at True Forged will go further than any research on the web. I've heard this design referred to as Super Sports and 702s. There's probably more pics of these wheels on SVT Performance. Here's one thread.

 

I'm glad you understand it all. Not everyone does. While certainly not a cheap alternative, True Forged makes a similar styled wheel and they can make it with custom offsets. Here's a thread on that wheel design. There website leaves a lot to be desired, but phone call to Steve at True Forged will go further than any research on the web. I've heard this design referred to as Super Sports and 702s. There's probably more pics of these wheels on SVT Performance. Here's one thread.

Are we talking quality wheels again? :wink:
If I didn't have Alcoas, I'd be running these. Good looking wheel.
I'm thinkng of having Steve make a set for one of the Terminators.