[Tom Vaught]

Friend has a 70 GTO and is looking to put the 17" torque thrust style wheels on the car.
(Ames sells them).

17" x 8" on the front and 17" x 9" on the rear. probably a 275 x 17 Tire on the rear.

Has anyone done this on their 70 GTO or LeMans vehicle? Will they fit or is he limited to 8" width all the way around?

Thanks in advance.

Tom Vaught

[JLMounce]

There's a decent amount of room back there. Not a complete apples to apples comparison but I've got 18x9.5's with a 275/40/18 tire on the back of my wife's 71 Chevelle. 0 offset, straight off the shelf, but does have disk brakes. There's PLENTY of room with that setup.

On that car specifically I could go 10.5" wide with a 305 tire in the factory wheel house pretty easily, but you aren't buying 0 offset wheels in that situation. You'd have a custom backspace.

Wife 71 also has an 18x8 with 245/45/18 rubber up front, again, no issues. Her car is lowered 2" all around.

The GTO should have similar amounts of space. It's always best to make sure first though. Get out there with some rulers and straight edges and measure what kind of space you've got back there.

[HWYSTR455]

Yes, the 9" wide fits on the rear, which is the same as the 9.5" wheel, as long as you have the 5" backspacing.

When a wheel manufacturer lists a '9.5' inch wide wheel, they are measuring from the outside lips, and for a 9", they measure inside the lips. In 99% of the time this is valid, unless it's a custom built wheel.

On the front, the limiting factor is the upper control arms, so best to just go 8" wide up there. Zero offset or 4" backspacing.

Depending on the tire, and the fender lip, you should be able to get a 255 up front, and the rear 275 is also common.

The rear, you can squeeze some pretty wide wheels back there, but once you go over 10" wide, available offsets/backspacing is going to limit you. I did squeeze 11s on the back of the 71 lemans, would have to lookup what I had exactly for backspacing, but yes, you can go up to a 305 in the rear. Problem is, finding a matching set of 305/255s can be a challenge.

In general, 4" outer offset, as in 4" from the brake hub, is about as far as you want to go. Don't forget to account for rear disc upgrades, which pushes the wheel out an additional 1/2".


.

.

[amcmike]

Originally Posted by HWYSTR455

When a wheel manufacturer lists a '9.5' inch wide wheel, they are measuring from the outside lips, and for a 9", they measure inside the lips. In 99% of the time this is valid, unless it's a custom built wheel.

I'd never heard that before. Not saying your wrong, I'm just really surprised.

Originally Posted by HWYSTR455

On the front, the limiting factor is the upper control arms, so best to just go 8" wide up there. Zero offset or 4" backspacing.

Do you mean 0 offset or 4" backspacing will work, or are you implying 0 offset is 4" backspacing (which is incorrect)?

Offset is based on inside the bead to bead measurement, whereas backspacing is based on the outside the wheel lip. A typical wheel has about 1/2" inside bead to outside lip. So 8" rim with 0 offset = 4.5" backspacing. A 4" backspace would be abot -12mm offset.





Most manufacturers will list both on their site (for non-custom wheels). They might round a couple mm in some cases (for example the USMAGS 8" list 1mm offset and 4.5" backspacing).

[HWYSTR455]

Yes. There are 2 different width measurements, Actual = inside lip to inside lip, Overall = outside to outside. And what I'm saying is that some list one or the other, the ones that add a 1/2" generally are using Overall method. (EDIT: And they just list 'Width', and not which)

Sometimes you can look at actual technical specs of the wheel to determine which is which.

Ok, a zero offset 8" wheel is where the mounting point of the wheel to the car is exactly in the middle, which would be 4".

As for how they measure to list the backspacing is still dependent on which width is being used, actual, or overall. BUT, the standard is to measure to the OUTSIDE on the wheel as is with overall width measurement.

Tire Rack:

https://blog.tirerack.com/blog/turk-...ng-backspacing

And I attached the pic as an example, that's given in the article.

I know it's confusing, but what I've posted has been my experiences, and oh boy, I can tell you, I was taught a number of those lessons multiple times. The hard way.

You also have to pay attention to tires, you have to look at section width, not tread width, when trying to squeeze as much meat in there as you can. Also have to account for tire squirm.

One gotcha is on leaf cars, sometimes the multi-spring spring clamps, that hold the stack together, have had the sidewall rub that on hard turns (tire squirm).

.

[HWYSTR455]

Another indicator as it pertains to the wheel width is recommendations for tire sizes. If you notice, the same size is always recommended for an 8 and 8.5.

.

[The Champ]

Originally Posted by HWYSTR455

Ok, a zero offset 8" wheel is where the mounting point of the wheel to the car is exactly in the middle, which would be 4".

BS is measured from the inner lip:



Zero offset on an 8" rim is 4.5".

Zero offset on a 7" rim is 4".

[HWYSTR455]

That example is a measurement to the outer lip, which is backspacing, to the inner lip would be considered offset. Look at the pics I posted, it shows the different methods for measurement. The link also explains it.

But yes, Backspacing is going to be more than the offset measurement.

.

[HWYSTR455]

So I should have said '4" from the inner lip to the mount point'.

I see my error, since technically, that would not be a zero offset measurement, as outlined by the way offset is measured.

So corrected, it should read:

"Ok, a zero offset 8" wheel (measured by Actual method) is where the mounting point of the wheel to the car is exactly in the middle, measured to the inside lip, which would be 4". "

.

[Tom Vaught]

Great info! So a pair of new 17" x 8" Torque Thrust wheels from Ames for the front
and new 17" x 9" wheels for the rear should be no issue for a 70 GTO with a 255 tire on the rear. Correct??

Tom V.

[HWYSTR455]

Yes, common for that model/body, and many run that combo, it's a 'safe' selection. (staggered 17x8/17x9)

You can run 275 in the rear with no issues. It's a 'safe' size.

You can run at least a 245 on the front, many run 255 on the front. The only issue I've seen with 255s on the front is that sometimes, depending on ride height, the tire may make contact with 2 of the screws in the fender that hold the inner, but you can safely remove those 2 screws if that occurs. No negative impact by removing those 2 screws.

.

[The Champ]

Originally Posted by Tom Vaught

Great info! So a pair of new 17" x 8" Torque Thrust wheels from Ames for the front
and new 17" x 9" wheels for the rear should be no issue for a 70 GTO with a 255 tire on the rear. Correct??

Tom V.

Tom

It all depends on the BS.

I run 245/50/17's on the back of my '64 with a 12 bolt rear which we know is 1" wider than the factory 10 bolt. 255/50/17's would have fit, but weren't available in the tire I decided to go with.

I have mine mounted on 17x7's with 4.625" BS. Allowing for the wider rear end, I could have gotten by with 4.125" BS and still cleared ok if I had the original rear end.

For a 9" rim, I would have needed 5.125" BS with an original rear or 5.625 with my 12 bolt.

I know that there is more room in a '70, but how much more???

If you look at the amount of clearance your friend has with his current combo, it's actually pretty easy to do the math to see what will work with different tire sizes and rim widths and diameters.

Because so many of these cars have been altered over the years, I always hesitate to say what will or won't fit. Best is to look at what your working on and do the math.

You're an engineer - the math should be easy for you...

[Gastiresandoil]

I've run 17x9 wheels with 4.5" bs before. The fronts were 255's and the rears were 275's. The tires were from Kelly-Springfield's

[amcmike]

He might just want to get a pair of 17x9s first, and test fit on the front for clearance. Running the fronts on the wider side of rim width for a given tire will actually make the front fell more responsive in a turn and give the driver better feedback. On the front, you want the tire section width the same as, or even slightly less than, the overall outside-to-outside rim width. On a 17x9 rim that would be roughly 10 inches. So even though a 245 (or 255) can fit both a 8 and 9" rim, with the 9" would be preferred.

Have him check for clearance to the control arms and the lower tie-rods in addition to fender and frame, and if looks like it's going to contact, he can always order the 8s for the front.

[amcmike]

Originally Posted by HWYSTR455

I've seen with 255s on the front is that sometimes, depending on ride height, the tire may make contact with 2 of the screws in the fender that hold the inner, but you can safely remove those 2 screws if that occurs. No negative impact by removing those 2 screws.

Or, if you're OCD like me , use this style of bolt which would give a little more clearance and less likely to damage a tire compared to the factory bolts:

which I did at first on my Firebird.

But since the one bolt that was the biggest problem really wasn't needed (the others were enough), I just popped in one of these that I had on hand to plug the hole:



Found them here:
https://www.amazon.com/GOOACC-GRC-03...gateway&sr=8-1

[Tom Vaught]

Originally Posted by The Champ

Tom

It all depends on the BS.

I run 245/50/17's on the back of my '64 with a 12 bolt rear which we know is 1" wider than the factory 10 bolt. 255/50/17's would have fit, but weren't available in the tire I decided to go with.

I have mine mounted on 17x7's with 4.625" BS. Allowing for the wider rear end, I could have gotten by with 4.125" BS and still cleared ok if I had the original rear end.

For a 9" rim, I would have needed 5.125" BS with an original rear or 5.625 with my 12 bolt.

I know that there is more room in a '70, but how much more???

If you look at the amount of clearance your friend has with his current combo, it's actually pretty easy to do the math to see what will work with different tire sizes and rim widths and diameters.

Because so many of these cars have been altered over the years, I always hesitate to say what will or won't fit. Best is to look at what your working on and do the math.

You're an engineer - the math should be easy for you...

If I can get him to put the rear of the car on jack stands at his house or come to my house then were can take some real measurements.

He just asked me to make the post on the PY Board for him.

Math will not be an issue.

Tom V.

[The Champ]

I didn't think math would be a problem for you, Tom...