hi bryan,
120/80*18 is exactly the same as 4.00*18 (and 110/90*19 is the same as 3.25*19). there are no fitting problems. for the 80's bmw (and newer) you can change this also in your papers - if necessary (in germany we have a lot of regulations about that). just contact your bmw dealer or email to bmw in munich.
look also at "tourenfahrer"-magazin (partner of hu), they tested 7 tires for gs and such kind of bikes in issue 11/2001. if you like i can send you a pdf of the article (in german).
i'm riding a "metzler enduro 4" (120/80) on 860 cc gs-like self-made bmw and had a lot of fun on moody forest tracks AND on well maintained mountain roads. it's now 3,500 km and still looking like new. the last "metzler enduro 3" (4.00) made it for 11,500 km, with a lot of fast highway riding.
long may you run
gerd
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