I just got a 1982 CB125S. Its got the original tires on it with 700 miles on them. I want to replace them. Tube type. I've looked at a conversion chart but some things don't make sense, like the 17" tire on the rear is really an 18", they stamp it 17" but its really an 18" cause thats what it measures out to. Huh? Any help would be appreciated in some ideas about sizes I can use. I think I'll be relegated to moped tires from the small amount of initial research I've done, and thats ok.

rims are stamped 1.40, at least the front is
Front tire-2.75 X 18-tube type
Rear tire- 3.00 X 17-tube type

It looks like from what I've seen that a 90/90-18 would fit the front. Conti.Kenda, and IRC all make 90/90-18's but matching rears or even any rears aren't made. Going a size bigger in width is ok with me, I just don't know the conversions. Like is 3.00 or 3.25 or 3.50 a 100/90 or a 110/90? Its not a peg scraper or touring bike or cafe racer, it only gets up to around 45mph, it won't really be in the rain and will mostly be used for commuting and leisurely rides on 2 lanes. Thanks.

To get the inch-size equivalent, take the inch size of the tire (2.75) and multiply it by 25.4 (mm per inch)...that will get you a rough estimate of the width of the tire you are considering...although, IIRC...a lot of these tires are still available from Honda...and they didn't seem to be a lot of $$$ the last time I checked...might be worth giving that a shot...good luck!

Modified to add:  Don't forget that as you increase width, it's a total measurement...a 10mm wider tire is only wider by 5mm on each side when mounted...

Thanks, math is not my subject, but I do know the dosage of pediatric epinephrine! I went over to Dennis Kirk and they have a few selections there that actually look decent, not replica side car tire designs from the 60's.