What size tire using 15x6 wheel?

ima thinking the height is gonna be the problem
The rimwidth will work with 195s to 205s
But the height might be a problem, depending on how low your front end is. I think 25ish might be pushing it.
A 225/60-15 might be ~25.6
A225/55-15 might be ~24.7
A 205/60-15 might be ~24.7
A 205/55-15 might be 23.9
A 195/70-15 comes in around 25.7
A 195/65-15 is about 25.0
you could fit a 225 on those 6inch wheels, but to get the tread to run flat on the road, you will have to drop the pressure too low for my cornering style; in a turn the sidewalls roll under, and the tire loses grip in a hurry
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Personally,I wouldn't be comfortable with so little rubber on the front of my 68 Barracuda. I have 235/60-14s(25.1 tall), on 6.75 wheels (also ~3.375bs), on mine and they are already too skinny for my driving style. I had to slow down for them. I run these at 28/29psi
I put 245s on custom 15x8 wheels on the front, but it was a lot of work to finesse it on there. After the tires wore off in a few weeks, I went back to the former 235/60-14s. Been running that size ever since.

On the stock front end parts, I think a 4" bs is the max, before the wheel weights start getting swiped off by the upper BJs.