How much of a difference am I allowed to have in tire size compared to what I already have?

Just going to add it seems it could maybe affect ABS/TPMS although the place I read this at didn’t say to what extent. I guess it is hit or miss depending the vehicle and how it reacts?

EDIT: It also may affect my speedometer and it does not tell me that if does how much. Like 1 or 2 MPH or will it be way off?

Still haven’t found clear cut answer.

A one-step difference in profile with no changes to width or rim size will have negligible effect on your speedometer and less than a 2% difference in your odometer.

More importantly, ABS and other electronics go by the speed of the wheels in relationship to each other, regardless. What that means is that those systems will function so long as all four tires match, even if you put monster truck tires or skateboard wheels on.

I rarely mount stock-sized tires; I shop by “rolling circle” (fancy name for circumference).

A generic P235/60/R18 is 739.2 mm tall. A generic P235/55/R18 is 715.7 mm tall. That means your speedometer will be reading 3.2% high. At 60 mph that translates to 1.9 mph. So you will be actually driving 2 mph slower than indicated (if your speedometer was perfect.)
Your odometer will likewise read 3.2% high so if you figure your fuel economy you will suddenly see a similar jump but that will not be real.
You can easily check this with your GPS and see that the math works.
Like what @jerv said, your ABS works on the difference in wheel speed. It will work fine if all the tires the same size.

Plan on getting a set of rims for winter only and leave the winter tires on the rims and just change rims and tires not tires.
Repeated rermoval and mounting of tires can wear on alloy rims and the tire bead.

So I just looked at his tires again the picture, I plan today or tomorrow to look at them in person. However I noticed there was no P, LT or any letters in front??

Mine don’t either, so I wouldn’t be too concerned.
P = passenger car
LT = light truck (or mid/full-sized SUV; same chassis)
I can’t imagine that size not being rated LT even if it’s not marked.

@jerv Ah ok, I wasn’t sure if it was a big deal or not wanted to know before I drove out there and looked at them.

I have a check list for reviewing tires. If you can add more please let me know.

Dry rot, cracknside of tires
Check inside tires
DOT
Uneven wear and tear
Press/push side wall in for cracks
Look for plugs
Look for patches
See if any print on side of tires is worn off
Use penny for test someone said I can use a stick too how does that work?

I’ve bought a few sets of snow tires “with only one winter on them”. I put that in quotes because I’ve looked at more sets than I’ve bought. Three sets were from a trusted neighbor while the others… well,those didn’t get bought because a quick visual raised red flags.

My personal technique is to double-check the size first; if the numbers are gone then either the tires are older than advertised or have been sideswiped. A quick press on the sidewalls will show if the rubber is still good or if it’s dried out. Last, a quick peek at the inside; patches and plugs are obvious from the inside. Snow tires generally have enough tread depth (deeper than regular tires) that you can tell from 10 feet away, and will generally pass a penny test even when half-worn. Uneven wear is likewise blatantly obvious on snow tires. You almost don’t need to look; just a glance. ,

So yeah, you pretty much have it.