I'm definitely not trying to resize, I want the original profile, just wondered if there was a direct conversion. I'm getting the feeling there isn't?
There isn't. Tire sellers want you to compromise so they can move stock. However, you are better off using the inch size tires that Honda intended.
When metrics were introduced, they added an aspect ratio number, which is the tire's height in relation to width. A 100-90 x18 has a tire height 90% of its width, 100mm = 3.9 inch tread width with a 3.54 inch tread height. A 110-90 x18 equates to a 4.33 inch tread width and a 3.93 inch tire height.
The inch size tire height was generally 95% (could be more) of tread width size (with some exceptions). A stock sized 4.00 inch tire would normally have a tire height of about 3.8 inches, but it could vary by manufacturer.
A wider than stock tire has the tendency to wear faster in the middle, as most (if not all) oversize tires mounted on narrow rims will have to be over inflated to maintain side wall rigidity.
When new, metrics tires still have that nicely arched tread crown, and work well. As they age and wear unevenly, the crown flattens and this leads to changed handling characteristics. Many don't notice it's changed as it is gradual, until they replace the tires and are thrilled with the "as-new" handling.
The stock size inch tires have the correct tire height, and fit properly to the rim allowing proper tire pressures. This allows the tire crown to wear more evenly and for it to keep its arched shape longer throughout the life of the tire and maintain the nice handling that the bikes were born with.
You may have to special order the correct tires for your bike rather than use what the local dealer stocks. But, it is worth the wait and they are still available for those that want what is best for the bike.
Cheers,
