How do you determine Tire size on a 4x4 tractor when going to Low Pro tires vs Factory tall ones?

I'm currently trying that...

I think I have to submit a DNA sample and a Positive Covid test to proceed any further...I'm making slow progress as they seem to call this a "Straddle Mount"..and pic shows an Up/Stack exhaust...something seems goofy.

New twist....Front axle has Planetary gears out at HUB......so even if I found Gear ratio F&R....Will it represent what it is to the tire or just the ring& pinion?

I'll continue to dig.

I'm not sure the tires on the tractor I'm looking at are correct.
They been changed to r4 fronts and huge turf rears.
I have yet to see the "factory" tires and wheels that apparently come with the tractor......Thats the ones I need to measure.
Owners manual is No Joy so far with any tire specs...still wading through it.... thanks Doc.

Finally found this....took forever.

Tire Compatibility Chart—5420 and 5520—Early Model Tractor
NOTE:
Rear wheels have cast disks.
NOTE:
The following chart details which front tires are compatible with an available rear tire. Underneath each rear tire will be the compatible front tires, either 2-WD or MFWD, along with the compatible tractor model.
-: Tire Compatibility Chart—5420 and 5520



-: Tire Compatibility Chart—5420 and 5520



-: Tire Compatibility Chart—5420 and 5520

I need Low pro replacements if I go with this machine.

Part of my concern is ....Is whats on it Now the correct height F&R to not damage the MFWD system.

Now for field work your supposed to have x amount of slippage......whatever . Whole big long chapter on that and how to determine "slippage"...Never knew it was a thing but JD says it is.

I'm not doing any field work...It will be loader work on gravel road type stuff...Its a half mile to any pavement and I see no reason to ever be there .

I never checked what size is on it.
I'll have to go through my pics and see if I got one.

The r1's are going to be too tall to deal with my hills here I fear.
I like the low and wide look and I'll fill all 4.

Story was The previous owner just used it to mow flat land..likely was in 2x4 but I want to know how close they got the combo as they been on there a while. Center of the rears are worn pretty good.

I found nothing in the manual about this Lead...but that makes perfect sense.
This Slippage was all about how much ballast to add to which end depending how much tire Spin /Slippage your getting pulling a ground engageing implement.
I'll see if I can find that page and drop it in here.

Checking for Correct Ballast
The best way to check for correct ballast is to measure amount of travel reduction (% slip) of the drive wheels. Under normal field conditions, travel reduction should be 10—15 percent.

Add more weight to drive wheels if slip is excessive. If there is less than 10 percent slip, weight should be removed.

Measuring Wheel Slip—Manually
M47166-UN: Measuring Wheel Slip—Manually

1. • Place a mark (A) on a rear tire which is easily observed (a chalk mark is recommended).
   • With tractor working, mark a starting point (B) on the ground at the place where the tire mark (A) meets the ground.
   • Mark the ground again where the tire mark (A) completes 10 full revolutions (C).
   • With implement raised, return in the opposite direction. At the second mark on the ground (C) remark the tire (D).
   • While driving the tractor along the same path (implement raised), count the tire revolutions required to reach the starting point (B).
   • Use the return tire revolutions count and Wheel Slippage Chart to determine slippage. 10—15 percent for two wheel drive or 8—12 percent for MFWD engaged tractors is ideal.
   • Adjust ballast or load to give correct slippage.
     NOTE:
     Available horsepower is greatly reduced when wheel slip drops below 10 percent.
     -: Wheel Slippage Chart
     

     WHEEL SLIPPAGE CHART​
     Non-Loaded Wheel Revolutions (Step 5)​	Estimated % Slip​	Recommended Action​
     10​	0​	Remove Ballast​
     9-1/2​	5​	Remove Ballast​
     9​	10​	Proper Ballast​
     8-1/2​	15​	Proper Ballast​
     8​	20​	Add Ballast​
     7-1/2​	25​	Add Ballast​
     7​	30​	Add Ballast​

MX,BAIP,PA3-19-19960110

Yes Sir.

I'm trying to cover my but in every direction on this...gonna have to slightly finance a chunk of it creatively.

Going back by in a day or so and it will be time to talk if its still avl.

Learned lots so its not a total loss if its gone.

I think I've settled on this model or Era/yr tractor.....It seems to fit what I desire in a bigger machine.

Never have investigated the actual engine yet........No ones jumped out and said anything bad about a JD 4cyl and I've yet to stumble on any talk about engine troubles.
Hoping for the best......