80 series vrs. 90 series

Have decided to run 100/90-19 on the front but with the rear, haven't decided whether going with a 80 series or 90 series 120 on the the back 18" wheel. Wheels are stock on my 77' KZ1000A. The 80 series tire ( Bridgestone BT-45's ) is an 1" shooter which would help with my foot placement.

Any problems running an 80 series profile tire on the back?

The 80/90 series are the height of the sidewall as a percentage of the tread width. A 130-80-18 (making sizes up here) is about the same height as a 120-90-18.

If the tread width is the same, the 80/90 makes no real difference if it will fit or not, just in the height.

No, it lowers gearing slightly. Only problem is finding a long lasting tire for these bikes that don't cost too much.
Steve

BT45 120/80H-18 25.56" Diam.
BT45 120/90V-18 26.5" Diam.

Right at an 1" difference. So probably 1/2" shorter in height at the rear of the bike. Does that look about right?

Yeah, it's just under 1/2" difference in height at the rear of the bike (12mm)

Shouldnt be that big of a difference, may have to fool with your gearing a little though

Bridgestone is phazing out the Battlax BT45.
I went to order a set last week for my Z1-R.