AJ/FormS
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It's a bit difficult to decipher butSo 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.
>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.
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>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.
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