AJ this pic is with 245 tires on, how much more can you go?
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In the old days, we just measured the tread width and whatever it was, that was the rim width as well. So 8 inches of tread took an 8" rim.
For performance I now use the minimum 85% rule; so a 245 x.85=208; and 208/25.4=8.2" rim, which rounds down to 8 inches, or up to 8.5.
So to answer your question, you can go zero more on your 8 inch rim.BTW, the rim measurement is between the beads and usually about 1" less than the outside dimension...... but I'm sure you knew that.
If you do the math; a 255 will fit on an 8 at 80% so that is still doable,albeit at a new lower tire pressure, and that will lose some stability in the turns.
In fact, you can put a 275 on an 8 and come in at 74%, again with a reduced tire pressure to run flat to the road. This is fine in a straightline, but for me, even on 8.5s it was way to tricky.
And here's a bit of trivia for you; a 245 is only a 245 on the checking rim that the tire manufacturer used, which is usually specified as 70% of the branded size. So in the case of a 245, this is; .70x245=6.75 which rounds to 7 inches. Ergo, a 245 mounted on a 7" rim will be ~245 in tire section. Or will it? The nail in this coffin is that the section width is rounded to the nearest 10 millimeter on the 5s. So still on that 7" rim, your 245 could be anything from 240 to 250. And it changes just a little with tire pressure. The only way to know what you have, is to lay yours flat, put a straightedge across the sidewalls and measure it.
And finally, the more sidewall bulge you have, the greater will be the possible tire roll-over in the turns. So at 70%, there will be a lot possible, while at 90%, very little. After 90% it gets harder to seat the beads, especially as the sidewalls get shorter.For example; a 275/60 mounts on a 10 with only a bit of trouble, but a 275/50 may require the big air blaster. The 70% checking rim for a 275 is a 7.5, but i would strongly advise against running it that way. The 70% checking rim spec is just a spec, not a rim recommendation. I recommend a minimum of 85% as stated elsewhere.
Just for your consideration; I run 295/50-15s on 10 inch rims. This maths out to 86%. To have even treadwear, I have to run 24psi. Which makes plenty of tire bulge, and plenty of tire rollover, even just merging onto a hiway. And on the 10" rim the section-width is over 12 inches. 12.6 if I recall, which is 320mm
But if you are asking about distance to the springs, then it depends on how much body side-sway your car gas, and how well you have matched the tire-size to the rim-size, to be able to run optimum pressures.
I moved my springs inboard and the tub inner wall is now my sidewall limiter. I have very little side sway and can run as little as 3/8inch. To the springs, I have run much closer in previous iterations.
I once saw a Barracuda in a magazine, that could just get a creditcard between the sidewall and the spring. And not running a track bar. But he had more room to the tubs.
In your case, I would measure to whatever car part is nearest to the inner sidewall, then subtract 1/4 inch.If it happens to be the springs, then, it looks like there is 1.5 to the spring, so that gives you 1.25 inch .Because I like those rims so much, I would have, before powder coating, cut those rims apart and added one inch to make them 9s and run a 9/.85x25.4= 270 section width on a 9 checking , not a 270 branded size tire, but a 270 on the 9" rim. I'm gonna guess this will be a 255 branded tire. the math says 255x.85/25.4= a minimum 8.5 rim, so a 9 will let you run a lil higher pressure. Then, a 255 is on a 70% rim or 7 inch. The 9 is gonna add about half the difference to the section-width so 255+(2/2x25.4)=280 section, well that is about 5mm per side wider than the target, so shrinks your 1/4 inch to 1.5mm, it's gonna be tight!
Now with 1.5mmin clearance, you will have, hopefully, pulled the outter sidewall in towards the spring as well, and you have a little room to run a spacer , if need be. When it comes to bs, always err on the side of too much, cuz you can use spacers.
Because I have heard of your engine plans, and your cornering style, you are gonna need the biggest tires you can fit on the back, sooner or later, together with a traction aid of some kind.
And finally, those tall rims may have special needs, ie low profile tires. So you will eventually have to do some searching if you want to install a wider than 245 tire. Because remember, the lower the profile, the less flexibility you have in the rim-size to tire-size mismatch, cuz it just gets too hard to install the mismatched set.
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