Tire size conversion: 2.5x18 to metric?

[DucatiBastard]

Alright so I'm replacing the tires on my Honda Dream 50R. Original equip are still available (Bridgestone Battlax BT39 SS), however they are NON-DOT tires. For this bike to pass MO DOT Safety inspection I need street legal tires. 

There are a few options out there, but I am not 100% sure of the new tire size.  Original size one the sidewall is 2.50 x 18.  Per the online converters that should be a 90/90-18 tire.  But converting inches to MM yields 63.5mm width.

Obviously there is more to converting tires from inches to metric. 

Also, I am not sure if this bike runs tubes (the wheels are spoked).  The air valves have nuts holding them in, is this a clue?  Nowhere on the tire does it say "Tube-Type" or "Tubeless".  I know the Sport Classics rune tubeless tires with a tube.

Anybody have any FHE? I don't want to order the wrong tires in suck an oddball size.
Any and all insight is greatly appreciated.
Thanks!

[Dirty Duc]

All the tube-type tires I have changed have had a nut holding the valvestem (and therefore tube) to the rim.  Spokers are almost always tubes.  I've got nothing on the sizes, though.

[Howie]

http://www.motorcycle-superstore.com/5/32/404/32666/ITEM/IRC-NR53-Front---Rear-Scooter-Tire.aspx?WT.ac=SLIsearch

http://ssl.delti.com/cgi-bin/rshop.pl?dsco=135&Cookie=froogle-CKzn9d3IurcCFXFnOgodiz0A3A&details=Ordern&typ=D-M69087&ranzahl=4&nichtweiter=1&pk_campaign=froogle

http://www.amazon.com/IRC-Universal-Moped-2-50-18-T10166/dp/B000OO7CZS

http://www.heidenautires.com/vintage-motorcycle-tires/k40

http://www.dratv.com/irc40l2x18as.html

http://www.bridgestone.com/products/motorcycle_tires/products/accolade/index.html

[DucatiBastard]

yeah, I figured as much for the tubes.

Thanks Howie, it does make more sense to get the original size, don't know why i had it set in my mind I needed metric tires.

[Howie]

Doh, I thought you were having trouble finding the original size.  Metric equivalents won't really exist, smallest metric I know of  would be 90/90, which would require wider wheels and most likely has a different profile and rolling radius.