20 Year old tires, what size are they?

I,m working on getting my dads motorcyle back on the road. It has been sitting in my garage for about twenty years. The tires are rock hard so I'm getting ready to change them but theold rear tire size isn't crossing over to the new metric size? Front is a 3.6 H 19 that seems to cross over to a 100/90 19. The rear tire is a MT80 18. From what I have found it seems to be between a 120/90 18 and a 130/90 18? I believe stock is 110/90 18 but if the larger size fits I would like to use it. The wheels are stock spoke type. Can anyone tell me what size matches what is on the bike now? Thanks!

Go to the Home Page on this site, and check out Bike Specs.
I think the '76 KZ900 took a 4.00 x 18" rear tire. that means 4" wide, and 18" diameter. If you're looking at tires that just have metric sizes, then you'll have to cross reference. Most websites for m/c tires has a conversion chart, I think.

I have had both a 130/90 and a 110/90 on the rear of my 900. The next time I put a rear tire on it will be a 120/90. The 130 fits but you will never use the outer edges of it,(Lord knows I have tried but the pipes and pegs touch well before you get to the edge!)That would be a waste of money and rubber. The 110 is the "stock" size but I feel the RPM at freeway speeds is too high and I lost fuel mileage. The 120 should be a good balance between the two. BTW, I have the stock pipes and a 520 chain conversion with the stock ratios.