I'm in the market as well - in fact this will be a purchase in the next couple of days if I can just get some things resolved in my mind...
I was under the impression from reading a bit that it is better to stick with the same width front and rear - i.e. if 100/90 in front then 100/90 in the back regardless of rim size...
no?
Why would you think that? Seems pretty clear that Honda thought a larger rear than front served the bike better and handles well for street use. They choose tires to fit the rim width (or perhaps the other way around).
Stock size metric equivalents would be 82.5 (3.25) fronts and 101.6 (4.00) rear. Thats about a 20mm differential. I'd go with a 90 Front and 110 rear for near stock characteristics. If your tire manufacturer says their tire is appropriate for your rim width, then go with an equivalent differential in a larger width tread of 100 F and 120 R.
Lets say you are heeled over in a high G turn. On the edge, if the tires are at their traction limit/loading, wouldn't you want both tires to slide rather than just one of them? (directional stability) If your weight distribution front and rear were identical, same size tires front and rear would be okay, provided you didn't add power or braking while near the edge of traction. However, I think the SOHC4's weight is biased toward the rear, plus you are likely to add power or engine braking. These factors would require a larger tire contact patch on the rear of the bike to maintain the same relative adhesion characteristics as the front.
This all assumes front and rear tires are of the same profile and rubber compound, and you have not changed the bike weight distribution with rider position (clubmans for example).
Another factor is tire diameter. Smaller diameter tires have a smaller contact patch than equivalent width larger diameter tires. Stock tires are 19 Front and 18 Rear. The rear should be wider than the front for this reason, too.
Just rationalizing...
