Need recommendations for 75 TA, want wider Rally II on back with 275/60/15 M/T
 

A friend is building a 75 TA and trying to hook up somewhere around 650HP but drive on street and look stock. He has 9" rearend (3.70) and caltracks already.

He wants wider Rally II wheels but not sure what size/BS to get.

He wants to run 275/60/15 on back, maybe M/T street/strip tires.

Any suggestions from someone who has experience with this type thing.

Thanks for any help.

I had 15X8 rally IIs from Wheel Vintiques with 4.5 inch backspace. Had 275s on them and they were close but they fit my 72 Formula. No guarantees. I have stock 15X7 Honeycombs on it now with 275s and they clear and the car is pretty low. The sidewalls are sucked in a little, but I don’t care. Don’t know if that helps you. That setup is on the car in the avatar.

I was going to suggest the 15x8 wheels as well. I have 275’s on them and they fit good

Same here 15x8 4.5 backspacing 275/60/15 tires

I have a pair of Wheel Vintiques 15 x 8 Rally 2's that were custom ordered with 5.25" backspace to centre the wheel in the wheel well of my GTO with 275's and rear disc brakes. When I mocked up the same tires on a set of 15 x 8 Rally 2's with the standard 4.5" backspace, the tires ended up too close to the quarter, partially due to the disc thickness compared to drums.

When I figured out that I needed more backspace, I called and emailed WV and they would not give me the time of day...someone here suggested I call Summit and when I did, they confirmed they could do a custom order for me. This was about 8 years ago, no idea if they still do this.

I realize this does not answer your question about F body fitment but thought I would relay that custom backspace was available.

76 rally 2 15x10 4 1/2 bs with 275/60 stock 10 bolt ,love the look and stability around turns,hooks from a slight dig full throttle thanks to the 273 gear

Use the Tire Rack website to see section widths and corresponding measuring rim widths for the size you want to use. Click on specs and the dimensions are clearly displayed.

https://www.tirerack.com/tires/tires...9xxi9I7oocJ2Sx

A 275/60-15 will vary a little in size between manufacturers but I saw one that on a 8” wheel has a 10.7” section width. You might be able to find the exact tire brand with specs that you want on the Tire Rack, it’s a pretty solid tire reference site.

So use that overall section width dimension to determine how much room you actually have from the inner fender to the outer fender lip.

A straight edge and a tape measure is all you need. You can use a bare 15x7 Rally II wheel to measure off of to help you, mount it on the rear axle and set the car down on the bare rim using a block of wood under the rim. That way the weight is on the suspension and you can measure accurately.

Measure a lot, you’ll figure it out with a little effort. I would never trust anyone’s measurements but my own and so should you.

This is what you want. ^ 15x10" rim. The 275 tire will look suck in on the sides with an 8" rim. They fit much nicer on a 10" rim. I have the BF Goodrich T/A on a 15x10 " rim with 4.5" B.S. IMO
Like Bart says, you almost need to do some measuring. I do run a Moser 12 bolt rear-end. I did run the same tire on my 10 bolt rear-end with 8" rims and then moved up to 10" rim, same back space.

If you run a wider tire on a narrow(er) wheel, that 'bulge' takes away from what you can fit, not to mention it's not safe. It is possible for the tire to roll off the wheel. Also, on a dip or bump as the suspension compresses, the bulge can contact the lip and blow a tire.

If you run a 'bulge', you actually can only fit a narrower tire than without a bulge.

An option that helps is you can roll the lips, but more prefer not to do that.

Several things need to be known before trying to fit the widest possible tire on a 2nd gen F:

The 'baseline' wheel size on a 2nd gen F is 15x8 with zero offset. Or, in other terms, 4" back space. (EDIT: or 4.5")

Some wheel manufacturers measure width differently, so it's important to know that.

The distance to the wheel lip from the rear end flange is closer to the lip on the passenger side than the driver's side. So measure both sides.

When doing offset calculations, understand it's not just the fender lip that you need to watch, it's also the leaf. In some cases, the leaf buckle.

Tire 'squirm' can make contact even though they clear at stand still. Like around turns.

In general, you can't go more than 4" outboard on the wheel due to fender lip contact. That accounts for squirm/tire roll. (and no bulge). So when going to an aftermarket wheel, changes to width should be made towards the inside.

On like 74-up, there's also the seat belt bulge that can make contact. Might be '75-up or so.

b-man is correct, 'section widths' do vary, and you have to check/take into account. Keep in mind that measurement does not take into account for bulge when running a tire on 'unapproved' widths.

You should be able to do a 275/60-15 on a 15x8 wheel with 4.5 backspacing. A 15x9 with 5 or so backspacing would be a better choice though. And, it will depend more so on the tire.

That is not taking into account if you added disc brakes to an OE width rear.

The Herb Adams VSE catalog (should be able to find one) outlines much of this and is a good reference.

Just to say, it is possible to go too wide on a tire. As the contact patch increases, the corner weight gets distributed over a larger area and will not have as much downforce.

Just look how 10.5 cars can hook so well.

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Thanks for all the input. I'm passing it all on to him so he can figure out what is best.


I know he doesnt want to alter wheel lip.

Plenty of power, just has to try to hook it up.

Here you go,10" wide rim 275/60/15 MT drag radials.No way will that lip hit the side wal bulge.

I had 295/50R15s on my old bird with CI 10" rally iis outback if that helps...obvious bit of a rake on the old girl lol


https://bnz06pap002files.storage.liv...328&height=996

https://bnz06pap002files.storage.liv...328&height=996

Both those cars are way too high in the back, of course they won't hit the lip, you could go to 12" wheels on those and not hit. Those things stick way out past the quarter.

Go ahead and have 4 people in the car and see what happens.


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As HWYSTR455 says. If you jack the rear end high enough, you can put any width tire on the rear. But a 275/60/15 is over 28" tall - taller than the OE tires - that actually fit inside the fender.

Both photo examples look to have 4+" of space over the top of the tires.

The last set of 275’s I had were Hoosier DR’s and they actually measured just under 27-3/4” OD with just over 11” SW and just under 10” tread on a 8” rim.
After 1500 street miles and maybe a dozen passes at the track they were toast, but man do they hook!

Haven't tried putting 2 people in the back ,would imagine it would get closer to the fender,but the op friend is making 650hp and it is important to get the most traction to the ground.Airshocks were swapped out by a lot of owners of these 2nd gen f bodies to get clearance, inwhich even by today's standard its not a bad idear

I use Nitto 555 Drag radials on the street and at the track M/T et pros. I'm currently just using 8 inch wide wheels. A good suspension will help your buddy ..Caltracs and Split monos on rear and good shocks front and back. Also when comparing tires the ET Pros are a big 275/60 tire .

As the rear ride height rises, weight on the rear tires shifts to the front tires, and there's less downforce on the rear tires. By raising the rear, you're making any traction issues worse.

From the factory, 2nd gen Fs actually sat lower in the rear than the front. Can you guess why that is?

Again, reread my original post:

"You should be able to do a 275/60-15 on a 15x8 wheel with 4.5 backspacing. A 15x9 with 5 or so backspacing would be a better choice though."

There's no reason to jack the thing up with the correct offset and width. And people did that back in the day because of lack of understanding, and to 'be cool' with the jacked-up backend look.

Why not go to the track and look at similar cars that are hooking, see what makes them successful as a guide?

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Yeah tried the 8" wheel with 275/60 MT hooked up good but not as good as the 10"also there is a lot more rear end lateral floating (side to side movement)with the 8".No need to re read a post

My '70 TA pictured here with Nitto 275/60R15 drag radials on the rear.

Rear wheels at the time were American Racing Torq-Thrust in 15 X 8.5 ( p/n 105-5861 -24 offset 3-3/4 backside )

(Since then I have changed to Vintage Wheel Works 15 x 8, 3-7/8 back spacing on the rear and 15 x 7 on the front)

Front tires pictured are Firestone 215/65-15 with a 15x6 wheel, however later I changed to a larger 245/60-15 in front for the additional foot print for braking.

Rear suspension: Landrum 300 lb/in leaf springs, Edelbrock shocks, CalTracs traction bars, custom subframe connectors


https://www.motortrend.com/uploads/s...75&format=webp

https://www.motortrend.com/uploads/s...75&format=webp




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It was the style at the time...

https://th.bing.com/th/id/R.a24acf91...pid=ImgRaw&r=0

Clarence is always an issue...especially with the wagon trails we have for roads around here...good luck!

Nice Steve, always enjoy seeing pics of your car!

Bet the 300# leafs help control squat!


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Thanks.

Help control squat and no rear sway bar is needed.

At the time I purchased the CalTracs Traction Bars those were the Landrum leafsprings John Calvert was recommending with their use.
Now today today its Calvert’s Split Mono Leafsprings.


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1970 called….. :D

Seriously guys anyone can jack up the rear suspension to the moon and anything’s gonna ‘fit’.

It takes some careful figuring to get something that actually does fit without having to raise up the car.

Landrum Performance Springs / Technical Information


https://landrumspring.com/leaf-sprin...l-information/


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I need to find a picture of my car launching, I have Calvert split monos and Caltracs and the rear body of the car actually rises as the rearend is forced down. I may have a video somewhere.

Comments from this thread might be of interest....

https://forums.maxperformanceinc.com...d.php?t=864002



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Pretty sure my multi-leafs are #220 but would have to check.

Wasn't sure if they were going to be too much but am now thinking not!


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Thanks to everyone for all the help.

His car won Best Muscle Car at a recent show.


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Very nice! Post a pic!

Need recommendations for 75 TA, want wider Rally II on back with 275/60/15 M/T
 

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More pics

Great info here :beerchug:

It’s surprising how big of a tire you can cram into a f-body. This is a 30x12.5 ET Street biasply tire.

That's how you tuck a wide tire! Not the 80's stink bug stance with the tires hanging 2 inches outside the fender lip. I did that with my car in high school (1985)! Haha

ps. It tucks a 295/40/18 on an 18x10 now fwiw.

That's my current setup, 18x10 6" backspacing, 295/40/18. It will fit a 305 but I couldn't find a matching front tire for it. If I could have gotten an 18x11 with 7 - 7.5" backspacing I could have went with a 325.

This is with no weight in the car, it was gutted, but gives you an idea.

I did roll the lips, and have a 265/40/18 on the front with a 18x9 with 5" backspacing, and could have gotten a 275 up there, but couldn't get one in the series that I wanted.

The tires are from like 2002, so I will be shopping once I get it back from paint jail. Should be in a few weeks.


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You meaning Hub flange to wheel opening lip? My TA, the drivers side sticks out further from the wheel lip than the pass. 275/4518. But have had to roll the DS rear inner fender to minimze tire rubing Pass side fine. Is that common for them (axles) to not be centered in firebirds?

My understanding is in order to maintain the same axle length on both sides the housing was offset to the passenger side, but maybe I have the side mixed up?

The reason it needs to be offset is due to the pinion to ring gear relationship. The offset allows the centerline of the pinion to be on the same plane/centerline as the rest of the driveline.

So the side the ring gear is on the housing is further to the outside the body.

The choice GM had to make was, make 2 diff length axles ($$$) or offset the housing.

Others may have more detailed info on this than I do, but i came across this at one point trying to fit the largest wheel/tire combo as possible (in the early 80s).


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Yup, I got the sides mixed up...


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Very common with 2nd generation Camaro and firebird where the rear-end is offset to one side.

Ride height topic for 2nd Gen if interested....

https://forums.maxperformanceinc.com...ht=ride+height

( AMA Specifications in my post #10 )

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Just so you know, a 15x10 with 5.5” backspacing fits as well. I’ll try to get a picture of how these look on my car. They are Billet Specialties Single Bead Lock Win Lites. 15x10 5.5” BS rear with MT 275/60/15 ET Street R Radials.

The fronts are 15x4 1.625” BS. I did some research and the 15x4 2.25” BS sucks the wheel too far into the wheel well for my liking. I even spoke to tech at Billet Specialties and they said the same and recommended the 1.625” BS.

Here are 315 / 35 x 17 rear tires on one of my sons.

Yes, with proper backspacing it is possible to get 315s in a 17, or even an 18. In general, it requires an 11" wide wheel.

It gets tight in there, and most would be more comfortable rolling the lips. The more narrow a wheel is, the more the sidewall bulges, and a side load can make it contact the lip (not good at speed, will slice the sidewall). And more bulge allows the tire to roll more side to side.

There is only so much space between the leaf and the lip, so even if you narrow the rear you are still limited.

If you do strip launches, and make it soft (shocks & springs) for weight transfer, you have the same issue as with side load. Lower tire psi and the transfer will make the tire bulge more. When that happens is debatable, but you can't deny it won't.

And raising the car has it's drawbacks too, as already mentioned. It a fine line to walk.


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Nice car by the way, solid roof car too.


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looks sharp tho...trade offs galore

My friend with the 75 TA said a pic of his car is on the Robmcperformance website under custrides.

Cool!

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Just a heads up for anyone considering the Billet Specialties Win Lite wheels… My rear Aerospace Components pro-street discs do not fit inside the Win Lite wheels. They hit the caliper. Billet Specialties race wheels have a smaller inner barrel than most race wheels. They require a brake system with smaller than a 13.150” internal diameter.

I will be swapping out the rear brakes with an Aerospace Components pro-street low profile kit.
https://shop.aerospacecomponents.com...3-1-1-2-2.aspx

Figured I’d share my experience

I pulled the Weld drag lites and measured the backspace. They appear to be a 15x8 4.5” BS. I also discovered the inner part of the 30x12.5 MT tire was rubbing against the tailpipes. If you are thinking of trying this tire size you may want to consider something a little smaller.

New wheels and tires.

Top 30 of 300 at show.https://uploads.tapatalk-cdn.com/202...ce3ff942c2.jpghttps://uploads.tapatalk-cdn.com/202...0c8b1568fc.jpg

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Looks sharp

My TA has M/T DOTs 275-60-15 on ProStars 4.5 BS. They fit on a 8" "snowflake" that is about 4.5 BS also

Here's more information from the PT crowd.

https://www.pro-touring.com/threads/...irebird+wheels