There are a lot of options, including 3.55, 3.73, 4.10...
Look at the equipment identification tag under the hood, or use some math if you know your gear ratios
(5000RPM in overdrive [0.69] with ____ rear end, 15" 235/75R15 tires == 133mph.... if you do the math backwards you'd find 3.55)
18436572
16 inch
100 90 14
140/90-15 for rear and 100/90-19 for front.
rear of eng,by distribitor,Pass side
im not 100% sure bt i think the stock size is 215/75-R15
16" rear 19" front is the stock size on all xr 100 and xr100r
100/90 x19
130/90x16 rear and 100/90x18 front
Front: 100/90-18 Rear: 110/90/18
Front 100/80-17mc Rear 130/70-17mc
there actually is no ballast resistor on an 89 d-100. Ma Mopar left it out of fuel injected trucks, and 89's are all fuel injected.