OEM 2002 Bonneville America:
Bridgestone
L303 3.5 S18 Mag Mopus (110/80-18)
170/80-15 Exedra G546
Avon:
100/90-18 AM41 Venom - 32psi
170/80-15 AM42 Vemon - 36psi
Dunlop:
100/90-18 D404 - 31psi
170/80-15 D404 = 31psi
Metzler:
110/80-18 ME330 - 38psi (ME330 are no longer being made and have been replaced by the Lasertech)
170/80-15 ME880 - 40psi
A 100/90 is the same as 100mm x 90%= 90mm sidewall height,
while the 110/80 is 88mm in height,
so only 2mm height difference.
foot note contributed by Fishercat Tire size / Tube
conversion chart 
