I'm sure it's been asked before, I just couldn't find the answer so I figured I'd make a post. I have an 07 is350 lowered on S-tech springs which drops the car 1.2F 1R.. I'm going with a squared wheel set up. 18x9.5 +38. What's the best tire size to use that will fill most of the wheel gap? TIA.

 

Assuming you want to use same size tires on all 4 corners, 245/40/18 would be the closest size without rubbing in the front. You can go wider in the back using 255 or 265 if you choose to.
The 245/40 will give you very slight stretch (if any) on 9.5" wheel, which in your case should be enough to clear front fender tabs and fender liner at full turn.
Another thing to keep in mind is the shape of the tire. To give enough clearance look for tires with round shoulders vs, square ones, as this could mean rubbing or no rubbing at full turn. Also offset can play role here as well. Light rubbing can easily be fixed using a heat gun.

Use these tools to figure out your fitment.

Good luck

https://tiresize.com/calculator/

https://www.wheel-size.com/calc/

https://tyrestretch.com/

 

Easiest way to to figure this out is look for similar cars with similar wheel setups and see what they run. Search the forums, search the web the info is out there.

 

Appreciate the responses!

 

245/255 should do the trick. Make sure a tire that size doesn't cause any rubbing issues.