Its nice to see someone else modeling a Gradall
What model of Gradall are you making? If its a modern one like the xl4100v or the xl5100v the spec sheets have the tire sizes for the 6x6 and 6x4 undercarriages, the 6x4 uses a 385/65R22.5 and the 6x6 uses a 425/65R22.5 so the rim is 22.5" at 1/12 scale that is 1.88" (47.63mm) so a 1.9 tire is close to scale.
Here are the tire dimensions in 1/12 scale just in case you need them.
385/65R22.5
Diameter: 3.52" (89.32mm)
Width: 1.27" (32.17mm)
425/65R22.5
Diameter: 3.69" (93.77mm)
Width: 1.39" (35.35mm)
The "MIL-SPEC ZXL 1.9" tire from rc4wd will be a little over sized even for the 425/65R22.5.
Hope this helps.
Shawn