Service manual says back tire size should be 4.00x18. Does anyone have experience with or know if a 4.50x18 will fit?

My Thoughts--I think, or 'guess' it'll rub the chain guard or rub the arm. Look a bit over tired too. Bit like fitting a 350 to the front 19 on older models when 325 is original.

It might even start rubbing against the exhaust tail pipe first.

It won't fit! A friend of mine gave me a 4.50 X 18 and we tried it. It rubs. Stick with a 4.00 X 18.

The Perelli  Mt43  Is a large tyre,  with the trial tread pattern,,  in the 18x4.00  size,, ,, 

Yes, I've seen 4.00 tires that are wider than 4.50 tires.  (That's "tyres" if you're following along Gruncleal.. haha) 

So many inconsistencies in size between and even within brands...  seems to be much worse with the old "inch" sizing compared to the new "metric" sizing.

Thanks Mark - I always have my US to UK translation book to hand! 

Internet translation services are jolly useful when buying from Germany and France.