[Irwin]

I have a 1990 DR250S, with stock rear tire size of 110/100-18, will a 120/90-18 fit this bike?

Thanks

110/100-18 equals 4.5"/4.0"
120/90-18 equals 5.10/5.0"

right?

 

[DRider]

I think your conversions may be a little bit out.
120/90 = 4.7/3.5

divide mm by 25.4 to get inches