I am looking into some rear fender work for tire clearance. How much room do I need for tire clearance to be safe? Is it the same for inside v outside clearance?
Narrow body, no flares, 215-55/16 tires.

Go to Tacoma world tire calculator and punch in your tire size and the stock tire size. It will give you the actual tire dimensions and a side by side overlay. My 205/60/15 clear but just barely on full turn.

Not looking for size info, have done a bunch of that. Looking for how much clearance I need on the inside and outside of a particular tire so the tires don't rub anywhere in the rear of the car. How much will a tire generally move side to side while driving.

235 /35/ 20 on the gts 911 has zero deflection you could use those

What kind of answer is that? I can't run a 20 inch wheel on a narrow 914.

my rear clearance is with 16x7'' fuches and 205 /50/16,my guards are pulled a little and I have no rear trailing arm shims,I can get the tips of 4 fingers in the gap so I would expect you will need to roll the lips or cut a half moon in them with 215 's.
Good luck.

the amount of deflection would really depend on the tire brand and sidewall height...
a 50 series tire will deflect less than a 60 series... etc.

Not stuck on running the 215s, they came with the car. Still seeing what my options are. I don't think this was ever on the street, just a dream build for someone. Theory, not reality.

Split the difference, as stated I am running 55s now.

10-12 mm minimum would be a good starting point. Personally, I want more if I can get it. 12mm is less than the width of your index finger for most people - that isn't much.

When OEM does a motion study to check clearances they get a tire model from the tire supplier that includes all potential variation of tire dimensions within production tolerances (OD, width, lateral load deflections, what rim width it's used with, etc.) and also accounts for dynamic growth of the tire OD at maximum vehicle speed.

That tire model is then cycled through it's range of motion with some allowances for worn suspension bushings.

The 10 mm is a do not violate zone on top of the tolerances noted above. It is to make sure tires never contacts sheetmetal. I've seen the 12 mm violated to a soft contact with a wheel well liner which itself was still 5-6 mm from the nearest sheetmetal.

Now we are getting somewhere. So that on both sides? Gives me something to measure to. Thank you

All the way around in 3 dimensions.

The problem with what we do with hobby cars are there are so many unknowns.

The main thing you want to be concerned about is whether it clears at all ranges of suspension motion.

If it were me, I'd want to check by pulling the shock and spring and then cycling the wheel and tire though a full range of motion. That is a bit hard to do without also loosening the rear trailing arm pivots due to bushing wind up. You can probably get to full jounce using a floor jack. Full rebound won't be achievable without some method of pulling down on the trailing arm or loosening the pivot shaft.

Jounce travel is more important that rebound since the wheel / tire is going to be moving up into the wheel house where things get tighter to the sheet metal and fender lip.

To a degree you're going to have to just make sure you have room for deflections. Rims/hubs/bearings/trailing arms flex under max lateral load more than you would think.

I worry more about travel on my off road vehicle than I will on this, but I will check it. don't want surprises.

Same exact principle. Make sure it all clears at max travel. Most just look at curb height and assume it's all good.