'68 Newport 275/60r15 on 15x8 (4.25" B/S -6ET)?

[Metalmarty]

Hi all!

I'm looking for new rims and tires for my '68 Newport. It currently has stock 14" rims with 205/75r14 Hankook tires which are more than 10 years old and are not round anymore.

I've got my eyes on a set of 15x7 for the front and 15x8 in the rear.

I want the widest tires possible without (major) modifications. I've seen in multiple threads on the forum that 275/60r15 should be possible in the rear. But I'm not sure which backspacing I need to keep my skirts.


Will 275/60r15 work with 15x8 rims with 4.25" backspacing and -6ET? I want to keep my skirts!

Thanks!

 

[amazinblue82]

Hi @Metalmarty.

Not sure what "-6ET" means.

15x7 with 4 to 4.5 backsp, and a tire around 25-27 inch total diameter. No issues, unless wheels interfere with suspension/brakes for some reason.

Out back, as to 275/60-15's on 15x8 wheels, and backspacing between 4.0 and 4.5 inches, and you should be fine, skirts and all. I have done/using it on 4 different cars right now, it many times. An inch or so clearance or on either side should be left over. Nice aggressive look.

However, all cars might be little a different. Even same years/models. Just cuz they work on some other car, I like to verify it on my car.

As such, my personal caution is always measure your clearances (leafs on inside, body/skirts outside) at the same time before you get $$$ committed (like mounting wheels to tires so you can return if something aint workin' out, if you can.

Good luck!

 

[Metalmarty]

Hi @Metalmarty.

Not sure what "-6ET" means.

15x7 with 4 to 4.5 backsp, and a tire around 25-27 inch total diameter. No issues, unless wheels interfere with suspension/brakes for some reason.

Out back, as to 275/60-15's on 15x8 wheels, and backspacing between 4.0 and 4.5 inches, and you should be fine, skirts and all. I have done/using it on 4 different cars right now, it many times. An inch or so clearance or on either side should be left over. Nice aggressive look.

However, all cars might be little a different. Even same years/models. Just cuz they work on some other car, I like to verify it on my car.

As such, my personal caution is always measure your clearances (leafs on inside, body/skirts outside) at the same time before you get $$$ committed (like mounting wheels to tires so you can return if something aint workin' out, if you can.

Good luck!

Thanks for the information.
I think ET is also called offset?

"Offset is stamped on a wheel as an ET value, which refers to the German phrase “Einpress Tiefe.” ET indicates the distance between the mounting surface and the geometric centre of the wheel (the symmetry axle), usually expressed in millimetres.

When the ET value decreases, the alloy wheels will protrude further out. An increased ET value, on the other hand, results in moving the wheel location deeper into the wheel arch."

So the wheels I've found have a 4.25" backspacing and -6mm offset.

 

[amazinblue82]

I see. 6mm is about 1/4 inch, hence your 4.25 inch bksp. measure I think . On this side of pond, we use this convention.
An 8 inch wheel, with a "zero" offset would have 4.0 inch backspacing.

I am a little confused on the "negative sign" though. A "negative" offset here means the backspacing get's smaller ... all things equal, the wheel starts to move toward the "street" side of the car.

Is that the same in Europe?

Exactly What Wheels & Tires Fit C Bodies?
rim size and offset for newport

You still should be good, subject to your best attempts at measurements before you get committed to avoid wasted time/Euros, if possible, to confirm your fitment estimates.

 

[Metalmarty]

I see. 6mm is about 1/4 inch, hence your 4.25 inch bksp. measure I think . On this side of pond, we use this convention.
An 8 inch wheel, with a "zero" offset would have 4.0 inch backspacing.

I am a little confused on the "negative sign" though. A "negative" offset here means the backspacing get's smaller ... all things equal, the wheel starts to move toward the "street" side of the car.

Is that the same in Europe?

Exactly What Wheels & Tires Fit C Bodies?
rim size and offset for newport
View attachment 588339
You still should be good, subject to your best attempts at measurements before you get committed to avoid wasted time/Euros, if possible, to confirm your fitment estimates.

Almost.
The J measurement is on the inside of the rim. The backspacing is to the outside of the rim.

So a 15x8 rim is 9" wide at the ouside. Which with 0 offset has a backspacing of 4.5". Which explains a 4.25" backspacing with a -6mm offset.

I just found this handy link on the summit racing website
Is backspacing related to wheel offset?

.

But good the hear that it should work!
I'm definitely measuring myself. But it's always good to hear some succes stories before trying it.

 

[amazinblue82]

Thats very helpful. Thank you.

If you get a chance, let us see the finished product!

 

[amazinblue82]

Almost.
The J measurement is on the inside of the rim. The backspacing is to the outside of the rim.

So a 15x8 rim is 9" wide at the ouside. Which with 0 offset has a backspacing of 4.5". Which explains a 4.25" backspacing with a -6mm offset.

I just found this handy link on the summit racing website
Is backspacing related to wheel offset?

.

View attachment 588340

But good the hear that it should work!
I'm definitely measuring myself. But it's always good to hear some succes stories before trying it.

one more thing i noticed @Metalmarty

Over here many of us learned to/do measure rim width "bead to bead": source: https://discounttire.scene7.com/is/image/discounttire/EDUrim-width_rim-width_hero_002?wid=375

"How to Measure Rim Width​

Whether you’re hunting down new wheels or just replacing your tires, it’s pretty important to know your wheel dimensions and how they impact your ride.

Sure, you’re familiar with wheel offset and wheel bolt patterns, but what about your rim width?

Also referred to as wheel width, the width of your rims is measured from bead seat to bead seat, as indicated in the image above. That’s because this is the point of contact between your tires and your wheels.

Luckily, measuring rim width is pretty simple to do! Just take a ruler, yardstick or tape measure and measure the distance from bead seat to bead seat. The rim width is also included in the wheel size, which you can find stamped on the back of the wheel, or in your owner’s manual."

---

---

To most of us over here, a 15x8 "bead to bead" wheel IS in fact a 15x8 wheel, stamped as such and IS 9 inches wide from "edge to edge"

But the beads seats on a tire that properply fit/sized to accommodate an 8 inch "bead width". "Zero" offset in that case, would be 4.0 inches

As you explained an diagrammed, what you said makes perfect sense .. offset and backspacing measured from the midline of the entire wheel. We do as well .. except midline is measured "bead to bead". Negative/positive references are still fine from a "zero" offset midline of 4.5 inches

Most of the time it doesn't matter .. except when stretching at the margins of fitment and therefore safety (good bead sealing, sidewall flex/contact patch of the tire, etc).

Our C's are quite forgiving with tire/wheel sizes that are most commonly used factory & aftermarket. Generous wheel wells by design for most "normal" sized tires on 14-15 inch wheeld.

I only added this because other people looking at this -- from either region -- may do things differently. My goal is not to make the anybody do it one way or another.

I just know what I learned to do and have seen done most often .. to avoid confusion I just wanna be clear on what I am looking to buy or sell, or giving an opinion to someone else.

 

[pomonamissel]

this was my 66 300 with 15x8's n 275/60-15's skits needed to be modified .

 

[Metalmarty]

this was my 66 300 with 15x8's n 275/60-15's skits needed to be modified .

View attachment 588474

View attachment 588475

View attachment 588476

View attachment 588477

Looks great.
What offset / backspacing do you have on your rims?

 

[pomonamissel]

no idea of offset bought and sold with the car years ago , rims were from stockton wheel i believe .

 

[amazinblue82]

no idea of offset bought and sold with the car years ago , rims were from stockton wheel i believe .

hey @pomonamissel

didn't we (the group) toss some opinions at your set up a few years back? Sucks getting old.. but I seem to recall your car here.

Anyway, just eyeballin' the rears vs front (assuming they are different and just not camera angle or something)., the rears look a little "deeper" in the face area, meaning the they could be 15x8 or 8.5 and//or less than 4.5" bkspc.

IF (that's a big IF since we don't know for sure) they were at the extremes -- I will just make up this hypothetical to a set up I know from experience that FAILED me ...

Say 8.5" wide, 3.75" bkspc. on 255/60 or 275/60 tire -- I predict such a set up WOULD NOT fit a slabbie without mods (sheet metal, leaf relocation, etc).

the 255/60 tire has a about inch less width vs. the 275/60, all things equal, but with that wheel size I think setup STILL FAILS without mods.

I am sure somebody could be driving around my parking lot with that setup right under my four eyes -- asserting an UNmodified slabbie -- I am simply saying I would STILL have my doubts on that being effective/safe let alone EVEN IF possible to start with.

Back in the real world, that's why I do/encourage others to measure first. No need to guess/speculate. Me? I need it to fit AND I need a clearance "margin" for body roll, cornering, etc. in use, particularly the further away from a factory spec I wanna go.

Thanks for this discussion guys. Again, good luck/be safe @Metalmarty.

 

[pomonamissel]

well i didn't own that car for long , but i did a rear end swap from a 67 b body in my 67 300 rag to center my vintage old school american torq thurts 8.5x15 which have a 1.25 inches of pos offset to widen the track stance the narrower r/e made my car body modify free . and i choose 245/60x15 on the rear and 225/70x15 on the nose , i agree measure measure measure first , then go for it .

 

[pomonamissel]

had the same setup of rims on my 64 300k with no changes to body or running gear and you can see the tire on the rear is only a 225/70x15 , but room for a larger tire . but this setup was on my 65 880 rag which needed the fender lips to get rolled up on the rear quarter if i had changed out the rear end like i had done on my 67 300 rag no mods would have had to be done .

 

[Dads300]

I’m gonna go out on a limb and say 275/60 will not fit on an 8” wheel unless it has 5” or more of back space. I am running 265/70r15s on the rear of my 66 300. They would not fit without rubbing the outer wheel house (forget about skirts) on 15x8 dodge Ramcharger steelies which are listed at 4” bs or -12 offset by wheel vintiques who makes repops. I ended up using a 15x7 wheel with 4” or 4.25” backspace ( it’s been a few years) and have great clearance and zero rubbing. The problem is that the sidewall bulge is where they rub. There isn’t a ton of real estate between the spring and the skirt mechanism. If you want to run wide tires, you’re going to need to do some careful calculating.
Travis..

 

[amazinblue82]

I’m gonna go out on a limb and say 275/60 will not fit on an 8” wheel unless it has 5” or more of back space. I am running 265/70r15s on the rear of my 66 300. They would not fit without rubbing the outer wheel house (forget about skirts) on 15x8 dodge Ramcharger steelies which are listed at 4” bs or -12 offset by wheel vintiques who makes repops. I ended up using a 15x7 wheel with 4” or 4.25” backspace ( it’s been a few years) and have great clearance and zero rubbing. The problem is that the sidewall bulge is where they rub. There isn’t a ton of real estate between the spring and the skirt mechanism. If you want to run wide tires, you’re going to need to do some careful calculating.
Travis..

From what you described, I tend to agree. You may have moved the interference to tire-on-leaf spring.

As you may have already/will soon, variancing off the 15x7 clearances, will tell you a lot. It could be VERY close but the standard "buldge" (people tend to forget about this on radials on find they have LESS space than they thought) of a radial sidewall on a 275 tread width might be too much.

My '67 300 (NO mods) has 15x8, 245/60-15, 4.5 bksp, has 0.5 inch clearance (less than I like) skirts on, between sheet metal on driver side (almost an inch on pass side - dunno why) and a tad less between TIRE sidewall buldge and leaf.

a 275/60, 15x8 4.5 bksp likely would NOT fit on my car, at least NOT on driver side. That 275-15x8-4.5 bksp combo is overall an inch or so WIDER whan with the 245 tire. even if 5.0 bksp hypothetically, radial buldge and/or body roll in operation could create interence(s) at Rear wheels.

 

[Dads300]

Like the 67 above, I recall my drivers side being the side with more fitment issues. As I recall, my backspace on the new 15x7 wheels was very close to the spec of the 15x8. The benefit of the 7” wheel is that it pulled the outside bead in 1” from where it was on the 8”, thus providing clearance for the skirt. As I recall there is over an inch at the springs and maybe 5/8” or 3/4” on the outside. As I said before, zero rub during any driving condition. Also, my car is on all original unmodified suspension
Travis..

 

[fury fan]

well i didn't own that car for long , but i did a rear end swap from a 67 b body in my 67 300 rag to center my vintage old school american torq thurts 8.5x15 which have a 1.25 inches of pos offset to widen the track stance the narrower r/e made my car body modify free . and i choose 245/60x15 on the rear and 225/70x15 on the nose , i agree measure measure measure first , then go for it .

That's what I did, 67 B-body axle and installed with an old DC offset bracket kit. (alternatively - could've put new spring perches on in the C-body location)
Used 275/60-15 tires on 15x8 Ramcharger wheels with 4" BS.
Fits nicely with the skirts, no rubbing.
Plenty more room inboard for wider wheel/tire before it hits the spring (again, due to teh springs moving further inward)