I measured he tire track*width ; used center of the left and right tires for front and back.
So, the rear tires are 55" and the front tires are 54".
If I go to 55" in the front, the wheel wells fill out nicely; I'm curious whether there is a rule-of-thumb for this
*edited, to correct terminology;

Wow...those are wide tires Jim!!

Tire Rack has specs on their website of the sizes of the tires they sell.

You might take a look.

You measured track, not wheel width.

No rule of thumb. Ride height, wheel width, wheel offset, tire width and tire aspect ratio are all going to play a role.

There are sure to be tradeoffs. Handling goal vs reality, interference with stock fenders (especially turning and going over bumps), the "look" you are after, speedometer error, etc.

Thanks for the correction, I couldn’t think of the right terminology. Post edited

Being the fronts, it would alter steering feel and scrub radius, but I doubt you would notice. It doesn't get drastic til you go with 1-2" spacers to fill out GT flares.