tire size question

[AJ/FormS]

So my tires are starting to show their age, and it's time to replace them. I'm currently running the stock size of 205/75/r15 on the stock 1984 M-body wheel of (I think) 15x7. IMO, there is not enough tire under the car, and I think it almost has the "pizza-cutter" look, especially in the back. Again, just my opinion. I'm going with the traditional BFG radial T/A with the white letters. I've been doing some research on different tire sizes and seeing what will work. This is what I found:

For the front, I'm thinking 215/70/r15, and that should be just a little wider while allowing enough room for full lock-to-lock steering.

The rear is where I have more questions. I'm leaning towards a 255/60/r15, and I took some measurements today and I'm pretty sure they will fit, but I don't want too much tire bulge on the sidewall. Does this seem like too much tire for the rear? I do like the fact that there will be more tire on the ground. Any input is welcome.

It's a bit difficult to decipher but
>The 215s will be just right on 7s, and
the 255s are about the widest tire one would put on a 7" rim and that is already on the narrow side. A 255 is only a 255 on a rim that is 70% of its branded size,
This means a 255 will be a 255 on; 255/25.4x.70= a 7" rim. This does not mean that a 7" rim is the correct rim size. IMO the rim should be closer to 80% or even 85% of the branded size.So then 255/25.4 x .8=an 8"rim .
> FYI
Now if you put a 255 on an 8" rim the section might grow a half inch or 13mm making your 255 into a 255+13=268..... So then your 255/60-15 ends up being a 268/57-15....... lol
But how about a 275? well the checking rim will be .7 x 275/25.4=7.6 which rounds to a 7.5 inch rim. But no one would run a 275/50 on a 7.5 rim, there is no way the outside treads would ever wear properly. On a 50series the .85 factor would get you a 9.2 inch rim,rounds down to 9". Now this is 1.5inches wider, so the section width might grow half of that or 19 mm. Adding 19 to 275, gets you an actual section of 294 or 11.6 inches. I tell you this because if you are expecting a section of 275/25.4=10.8, but on a 9 it is 11.6, OOps surprise it might not fit in the wheelhouse, with the backspace you ordered.
____________________
>If the rim is too narrow for the tire, it will pull up the outside treads and wear off the centers. To combat that, you will have to drop the air pressure, to try to get the outside edges onto the pavement. Maybe you win and maybe you won't. Maybe the tire pressure gets so low, that handling takes a dump.
>If the rim is too wide for the tire, it will press down on the outside treads, and you will have to increase the tire pressure to try to get the centers to run flat on the pavement. Maybe it requires a lot of pressure, and the old girl starts riding like a truck.
>I have found the 80/85% number to work fairly well for me.
Using that formula, a 255 will like an 8, a 275 will like an 8.5 to 9, and a 295 will like a 10
I run 295/50s on 10s and they are barely wide enough. To get decent tire life out of that combo, I have to run the pressure down at 24psi,which made cornering a little interesting until I got used to it.
Higher profile tires can get away with a worse mismatch than lower profiles.

 

[Jonnyuma]

It's a bit difficult to decipher...

That's a lot of information there, thank you.

 

[Jonnyuma]

Rims are stock 15x7 4.5 backspace.

Hears mine with a 80 Dip with the same size rears, but the Dip has 4 inch backspace
View attachment 29427

That 80 almost makes me wish I'd kept looking... almost. It's a real toss-up for me which body style I like better. The 77-79s are so sculpted but the 80-up just looks like business. Both are great-looking cars, and your picture and info tells me exactly what I needed to know.
Thanks, Doc.

 

[rcmaniac791]

It's a bit difficult to decipher but

Thanks for all the info! From what I gathered, (and please correct me if I am wrong) the 255 is the widest tire I can put on these rims before I have serious issues with fitment, and I may have to change the tire pressures so they wear more evenly. Also, the 215's on the front will work just fine.

In your opinion AJ, should I go with a more narrow tire in the rear, like a 245/60? or are the 255's ok since I'll be mostly be just cruising around and doing highway driving?

 

[4speedjim]

Ha Ha... Yup AJ gives us the 360* view!

That's a lot of information there, thank you.

 

[AJ/FormS]

Thanks for all the info! From what I gathered, (and please correct me if I am wrong) the 255 is the widest tire I can put on these rims before I have serious issues with fitment, and I may have to change the tire pressures so they wear more evenly. Also, the 215's on the front will work just fine.

In your opinion AJ, should I go with a more narrow tire in the rear, like a 245/60? or are the 255's ok since I'll be mostly be just cruising around and doing highway driving?

That's a tough call for me. Tires affect the appearance so much that really it's a personal thing.
A 215/75R15 is about 26.85 tall, so you probably wouldn't want anything shorter in the back.
A 255/60R15 is about 27, so appearance wise it works. And it works on an 8" rim. If it physically fits back there with the appropriate backspace, I like the combo for your application..... but I would not run it on a 7.
If you work the formula backwards; (7x25.4)/.8=222, and (7x25.4)/.85= 209; so a 7 will work well with 209s to 222s, ideally then, a 215.5,lol.
An 8 likes (8x25.4)/ .825= 246 +/- 10mm
I think my old DD is running 255s on 8.5s @29psi. Seems to be working pretty good.
The next size down, 245s are 28.6 tall as 70s, and are 26.6 as 60s.The 245/60-15 will love the 8s

 

[Jonnyuma]

This site may help
Tire Size Calculator
It gives you not only numbers, but an actual visual reference. I use it instead of a calculator when deciding on staggered front to rear tire/rim sizes.
Try it... it's FUN

 

[4speedjim]

Makes it a lot easier to make a decision. I use that one and tire racks frequently.

 

[Oldiron440]

Your living hard and fast by your tire specs AJ. The fact is the printed spec for a tire is more than likely incorrect and some manufacturer are worse than other's. I have a new set of Cooper 275/60/15 mounted on 8" wheels that are a full inch shorter than the spec for the tire.
I have found that this is a common problem with most tire co. and buying drag slicks a pita where you can have two identical tires with a large difference in rollout.

 

[rcmaniac791]

This site may help
Tire Size Calculator
It gives you not only numbers, but an actual visual reference. I use it instead of a calculator when deciding on staggered front to rear tire/rim sizes.
Try it... it's FUN

That's what I've been using. It's really cool.

but I would not run it on a 7.

What do you guys think of having the 215/70's on the front and then sticking a 225/70 on the rear? It's still a bit wider than the stock size while not really being so much bigger that it would affect the handling?

 

[Jonnyuma]

Makes it a lot easier to make a decision. I use that one and tire racks frequently.

I especially like that it gives the sidewall height.. that can be a pain if your mixing aspect ratios and/or staggering rim diameters.
I also like the feature that lets you convert from metric to standard sizes. It was very helpful on my 2wd truck... i knew I wanted a 30x11.50 tire, but have 16" rims... no such tire exists in the non-metric world.

 

[DCAspen]

I'm using a 245x60x15 rear with a 215x65x15 in front on 7" wheels.

 

[AJ/FormS]

Your living hard and fast by your tire specs AJ. The fact is the printed spec for a tire is more than likely incorrect and some manufacturer are worse than other's. I have a new set of Cooper 275/60/15 mounted on 8" wheels that are a full inch shorter than the spec for the tire.
I have found that this is a common problem with most tire co. and buying drag slicks a pita where you can have two identical tires with a large difference in rollout.

You are right, but it's all we got.
You are right because everything is rounded this way and that. The section with is rounded to the nearest 10mm, and the profile to the nearest 10%. All we can hope for is that tires of same branding have the same roll-out, once mounted and aired up samely..

 

[AJ/FormS]

That's what I've been using. It's really cool.



What do you guys think of having the 215/70's on the front and then sticking a 225/70 on the rear? It's still a bit wider than the stock size while not really being so much bigger that it would affect the handling?

I don't actually see a point in that.
Can you do it; sure. Will it affect handling; not hardly. Can you rotate the tires; yabut.

 

[AJ/FormS]

I'm using a 245x60x15 rear with a 215x65x15 in front on 7" wheels.

And how are the 245s wearing on the 7s? and what tire pressure are you running?and

 

[Dr Lebaron]

I've had 275/60/15 on 7inch on the back for 15 yrs or more.
I run 32/34 psi.
245 looked too small.

Tire wear is really nadda, but they are old now, so I got some new ones to install.
Buy them in the winter when they are cheap.

Then again I'm not doing burnouts with 110hp, 7 .25 rear and huge meat.

 

[DCAspen]

Mounting my tires in the next month for my 1977 Aspen R/T.

 

[rcmaniac791]

Finally able to update this thread on what I chose:

Did a 215/70/15 on the front and a 225/70/15 on the back. Chose to go more mild, but I've gotten plenty of positive comments at shows, and I'm really happy with how it turned out, and that's what matters, right?

 

[89.Fifth]

Looks great! And yeah. All that matters is that you're happy with how it turned out.

 

[Jonnyuma]

They fill up the wheelwells very nicely... better than I would have expected by just the numbers... very cool.

Even more important than tire size is your wheel measurements... what size and backspacing did you go with?

Looking at your 5A is proof that a guy (or gal) doesn't always have to stuff the biggest tire possible under a car to make it look cool... an affliction I (and my bank account) suffer from.

Classy, cool, and practical, I'm diggin' it big time. Nice job, beautiful car... I love the color, it's (the gold) on my short list when the repaint happens.