[jimib]

Before I buy a ready-built rear (probably from Moser) and a 5 or 6 speed Tremec tranny; I think rims and tires should first be fitted to replace my original 14-inch rims and tires. From the threads I've read, I see mixed results of what size rim and tire to go with. My fear with a 17-inch rim and tire is that it may look goofy or just not right. So thus far, I'm inclined to go with a 15-inch tire/rim combo. Also, with a standard 5 or 6-speed tranny and a 3.73 rear (which I plan to buy), I can cruise on the interstate at a relatively low RPM and still have decent acceleration when the light changes to green. Lastly, my intention for this car is to be a cruiser, no track, no racing.

I would like to hear thoughts and opinions on your preference for tires and rims on your ride. Also, please elaborate on your rear and tranny.

[The Champ]

I will show you the exact same car with both 15's and 17's. '64 GTO Convertible with the original Muncie 4spd and I've had 3.23, 3.36 (both 10 bolts), 342 and now 3.08 gears (12 bolt).

The first two photos are with 15's - 235/60/15 front and 255/60/15 rear (driven this way from 1995 - 2017.

The second two photos are with 17's - 215/55/17 front and 245/50/17 rear.

I decided to go with 17's because I thought they looked fine and gave me much better tire options. I'm very happy with the 17's

[jimib]

Originally Posted by The Champ

I will show you the exact same car with both 15's and 17's. '64 GTO Convertible with the original Muncie 4spd and I've had 3.23, 3.36 (both 10 bolts), 342 and now 3.08 gears (12 bolt).

The first two photos are with 15's - 235/60/15 front and 255/60/15 rear (driven this way from 1995 - 2017.

The second two photos are with 17's - 215/55/17 front and 245/50/17 rear.

I decided to go with 17's because I thought they looked fine and gave me much better tire options. I'm very happy with the 17's

The Champ; thank you, this is a fantastic response. The 17's look just fine. You've answered my question. (-:

[gtospieg]

Here are pics of my 67 with 255/45/17 on 17x8 wheels with 5.5 backspace on all 4 corners.

[jimib]

Originally Posted by gtospieg

Here are pics of my 67 with 255/45/17 on 17x8 wheels with 5.5 backspace on all 4 corners.

gtospieg:
Your car is a 67 like mine. Your car is gorgeous and I like the slight rake. Your car is the look I want. The 17-inch rims/wheels look just fine just like our friend above showed us on his vehicle.

My 67 sags in the rear; I've got to fix this so it looks like yours. In another thread I started regarding "sag", I received a lot of great advice but I think the best idea is to install station wagon springs or there's a company that makes them custom.

"Backspacing"
This is a subject I have to study up on. Currently; I have 14 inch wheels, so moving up to a 17 inch and whatever width I decide, I think I need to know about back spacing.

[The Champ]

Backspacing isn't that hard to figure out when upsizing rims.

Let's say your current rims are 6" wide with 4" BS. It doesn't matter what diameter the rim is, a 6" wide rim should have the same BS as your existing rim assuming the tire width remains the same.

If you go to a 7" wide rim, you need to go to 4.5" BS to put the wheel and tire in the same position.

If you go to an 8" wide rim, you need to go to 5" BS to put the wheel and tire in the same position.

Now, if you're planning to go to a wider tire, you may want to go with more BS.


I prefer to keep the rear tire size as close to 27" (original tire height) as possible. My 255/60/15's were 27.1" tall, my 245/50/17's are 26.6" tall.

If you add disc brakes, they normally increase the need for BS by about .25".

I added disc brakes up front - my 15x7" rims had 4.25" BS and my 235/60/15's (26.1" tall) had a tendency to rub slightly under certain conditions after the switch. My 17x7" rims have 4.625" BS and I now have zero rubbing on my front 215/55/17's (26.3" tall).

When I made my switch to 17's, my grandson worked for Continental and I was able to use his employee discount to purchase my tires. This limited my tire size options and I had to order my tires through Tire Rack. The largest rear tire I could get through Continental was the 245/50/17 although I would have preferred a 255/50/17 (27" tall).

But I only paid about $400 delivered for my 4 Continental Extreme Contact DWS 06's - normally over $650 at Tire Rack.

[FrankieT/A]

Originally Posted by jimib

gtospieg:

My 67 sags in the rear; I've got to fix this so it looks like yours. In another thread I started regarding "sag", I received a lot of great advice but I think the best idea is to install station wagon springs or there's a company that makes them custom.

Back in the 80's I had '68 GTO convertible and I took rear springs out of a 65 full size Pontiac wagon and they made my stance perfect like gtospieg's, in the 90's I had an '86 Monte Carlo SS with a big block and once again I used the rear springs out of a full size wagon, and again had the same results and I would do that again, but you won't find much support on this forum for that.

[David Jones]

On my 69, I've found that it drives and rides much better with 70 series tires on the front.

[JLMounce]

If you are going to buy a new rear-end for the car, get that BEFORE you buy new wheels. Pontiac track widths vary from their chevy counterparts for which those new axles are going to be based around. Adding things like disc brakes further change total hub to hub distances.

Do those items before you buy wheels so that you can measure properly and not find yourself in a situation where you may have just put tires on a set of wheels that cost you a couple grand, only to find out that they don't fit.

[1965gp]

I’m a big fan of 17’s- we are so used to seeing 20 and 22’s that 17’s have just enough tire on them. I kept the original Rallye wheels on my 66 because I like the look on a 66-67. Still, if you are going with aftermarket wheels I would go with 17s.

Here is a pic of my 66 - I used the rear springs that Ames sells for the stance.

[amcmike]

Originally Posted by JLMounce

If you are going to buy a new rear-end for the car, get that BEFORE you buy new wheels. Pontiac track widths vary from their chevy counterparts for which those new axles are going to be based around. Adding things like disc brakes further change total hub to hub distances.

Do those items before you buy wheels so that you can measure properly and not find yourself in a situation where you may have just put tires on a set of wheels that cost you a couple grand, only to find out that they don't fit.

^^^this.

Even the Firebirds are wider from the factory than their Camaro counterparts. Luckily for me, when I swapped rearends I went from drums to discs at the same time and that basically made it a wash.

Which is another point. If you plan to change brakes, do both the rear and brakes before you fit new wheels.

[389]

Stay with the 15" wheels, the 18 wheels look goofy. The body wasn't made for those big wheels like the new cars. The handling and braking is in the suspension parts and brake lining material. Use Porterfield brake pads and matching shoes and it will stop on the dime..

[JLMounce]

Originally Posted by 389

Stay with the 15" wheels, the 18 wheels look goofy. The body wasn't made for those big wheels like the new cars. The handling and braking is in the suspension parts and brake lining material. Use Porterfield brake pads and matching shoes and it will stop on the dime..

This isn't completely accurate. The looks bit of the equation is subjective, but as far as braking and handling performance goes, the larger wheel package offers an advantage simply because you can take advantage of modern tires.

The most important performance part on a car is the tires connecting it to the road.

Suspension matters and so does brake linings, but it's meaningless if you can't transfer that to the road. The tires you can put on a 17" or 18" wheel are superior in just about every way to the tires currently available on a 14" or 15" wheel. In those sizes you are stuck with the likes of BFG Radials, Cooper Cobras or drag radials.

For the purposes of handling and braking, these cars are horribly undersprung and under-damped. Those large, soft tires like a Cooper Cobra also act as the suspension, further softening it. They allow the contact patch to distort and because the section width is carried in the sidewall and not at the contact patch, you lose further grip through a smaller actual contact patch.

Even on a completely stock suspension and factory brakes, a car with a 17" wheel with a modern performance all season tire will outhandle and outbrake that same car on old style tires. This will continue to be the case until somebody decides to provide 15" wheel size tires with modern construction and compounds.

[Scarebird]

Originally Posted by JLMounce

...Those large, soft tires like a Cooper Cobra also act as the suspension, further softening it...

...Even on a completely stock suspension and factory brakes, a car with a 17" wheel with a modern performance all season tire will outhandle and outbrake that same car on old style tires...

I was always amazed how mushy my 71 was - even with tall ball joints, 442 springs, HD sway bars, Konis and modern 15" tires (General Altimax).

[The Champ]

Originally Posted by 389

Stay with the 15" wheels, the 18 wheels look goofy. The body wasn't made for those big wheels like the new cars. The handling and braking is in the suspension parts and brake lining material. Use Porterfield brake pads and matching shoes and it will stop on the dime..

First of all, the original poster isn't looking at going to 18's.

Secondly, these cars came with bias ply tires, but most of us use radial tires today for improved ride and handling.

Thirdly, these cars were built with 14's, so even a 15" wheel is larger.

I've driven my GTO since 1994. Originally drove it with 14" 75 series radials, then 15" 65 series radials and then 60 series radials and now with 17" 55 and 50 series radials.

I've posted photos of my car with both the 15's and the 17's. The original poster stated that he likes the looks of my car with the 17's. Most people don't even notice I've got 17's until I point it out. Must not look too goofy....

If he likes the looks of a 60's GTO with 17's - then he should go that way with his car.

[JLMounce]

Originally Posted by Scarebird

I was always amazed how mushy my 71 was - even with tall ball joints, 442 springs, HD sway bars, Konis and modern 15" tires (General Altimax).

Yeah these cars are real soft when compared to modern cars. Not too many people in the market for cars these days desire a floaty disconnected feel from their vehicle.

[389]

Originally Posted by The Champ

First of all, the original poster isn't looking at going to 18's.

Secondly, these cars came with bias ply tires, but most of us use radial tires today for improved ride and handling.

Thirdly, these cars were built with 14's, so even a 15" wheel is larger.

I've driven my GTO since 1994. Originally drove it with 14" 75 series radials, then 15" 65 series radials and then 60 series radials and now with 17" 55 and 50 series radials.

I've posted photos of my car with both the 15's and the 17's. The original poster stated that he likes the looks of my car with the 17's. Most people don't even notice I've got 17's until I point it out. Must not look too goofy....

If he likes the looks of a 60's GTO with 17's - then he should go that way with his car.

So your telling me you drive a car with the original suspension that positive cambers on compression and you are running modern low profile stiff sidewall tires. Do you have any idea how stupid you look to me and anybody else who knows suspensions right now? The first time you try to panic stop that death trap your driving you will remember this post and what I was talking about.. I guarantee it..

[The Champ]

Originally Posted by 389

So your telling me you drive a car with the original suspension that positive cambers on compression and you are running modern low profile stiff sidewall tires. Do you have any idea how stupid you look to me and anybody else who knows suspensions right now? The first time you try to panic stop that death trap your driving you will remember this post and what I was talking about.. I guarantee it..

What do you know about my car?

Absolutely nothing.

You're the one that is looking stupid right now for making unfounded assumptions not backed by a single shred of evidence.

Carry on.

[1965gp]

I will say all of my cars seem to drive better with the larger tires. Even my 60 Wagon- much better on the 20’s vs the 15’s or 14’s that we’re on it.

[amcmike]

Originally Posted by 389

So your telling me you drive a car with the original suspension that positive cambers on compression and you are running modern low profile stiff sidewall tires. Do you have any idea how stupid you look to me and anybody else who knows suspensions right now? The first time you try to panic stop that death trap your driving you will remember this post and what I was talking about.. I guarantee it..

I'll bite. What is it exactly about mushy sidewalls that plays a major role in vehicle control during brake dive?

Especially since, even with heavy braking and soft spring rates, you probably wouldn't see much more than an inch compression. And a stock 1st gen Firebird for example, keeps the camber gain zero, until you start to go past that (even full compression it's still less than 1 degree positive). And if it's a combination of panic braking into a corner, that <1 degree maximum change is insignificant; when compared to the loss of tread contact due to the larger slip angles caused by a softer sidewall with lateral forces.

And keep in mind these cars originally came with bias plys, which have stiffer sidewall than a radial of the same section height. So it's not as big a leap stiffness-wise (if any), when going to a modern lower profile radial compared to what they originally came with.