RPM Calculator, actual Tire diameter question

With a non-loc-up automatic, you will never get the at-speed rpm to agree with the calculated rpm......... because the TC is slipping always slipping. It's a fluid coupling not a clutch.
Next time you are out cruising, make note of the tach reading. Then simultaneously feather the brake and gas pedal to maintain your cruise speed and watch the tach. Next increase your speed a couple of mphs then lift off the gas, again watching the tach as the mph comes down across the chosen cruise mph.
Don't be surprised to see 200 to 400rpm difference, or more with a looser TC.

On a sidenote; my experience with radial tires is that at hiway speed they don't grow much; at 60mph, an 85"tire is rolling along at only 5250x12/85=745 rpm. How much is not much, IDK. But if it was 2%, that would be .02x745=15 tire rpm.
On your dashtach with 3.55s , the calculated cruise rpm using the roll-out of 85" is 2646, and 2% would be 53rpm,can you read 2646 on your tach? I bet your needle is more than 53 rpm wide lol.But worse is your speedo error is likely greater than that, on account of 2% of the 60 mph reading is 1.2 mph. I bet your needle is 1.2 mph wide. So how can you resolve this?
IDK either. And to be blunt, why would I care....... jus asking

As to calculating the roll-out from the branded tire size, this is impossible.
Impossible because there is a really large amount of leeway in the branding procedure.
Take for instance a 235/70-15 tire. The 235 is the section width on the specified test rim of 70%. So if you mount this tire on a .70x235/25.4=6.47 wheel, it will measure somewhere close to 235. But we already have the first problem; how many times in your life have you seen a 6.47" rim? Or even 6.5 with rounding?lol. But let's ignore that for a minute.
So now you have this tire mounted on a 6.5, and it measures not 235. Suppose it measures 230 or 240, both of which can legally be rounded to 235. This number, together with the aspect ratio is used to calculate the tire diameter and then the circumference.
So lets say your 235 actually measures somewhere between 230 and 240, and the aspect ratio is 70.
Hang on a sec; aspect ratios are rounded to the nearest 5 on the 5s, so your 70 series tire could actually be legally somewhere between 67.51 and 72.49. You see where this is going?
After you best guess the diameter, along comes Mr. Pi, and says he will multiply your best guess error by 3.1416.
It's impossible, I tellya,lol.