Could someone recommend a tire that is the closet match to the original tires on a 1970 RT-1

I used Shinko's on mine. 

Heidenau K67 (as has been posted here many times)

There's a whole tire thread with pictures if you want more info.

Shinko's are nice tires but they don't make the right size tire for the front on the RT1. I use Heidenau K67's on all Yamaha's from 68-72 with the 3.25x19 fronts.

True on the 3.25's.

Shinko's are great tires for 73-81 Yamaha enduro's with the 21" front wheel. They are a bargin compared to the Heidenau K67's. Don't know why they don't make a 3.25x19. 

Dennis,
Great looking DT3! Looks like you got the NOS front brake cable on it? 

Yes I did and thanks again. I need to work on the paint on the
tank a bit more and see if I can get some of the scratches out
of the clear coat. Had to paint the oil tank and headlamp parts
but trying to save the original paint on the tank.
I too have wondered about the size from Shinko. Lord knows
there are enough of the bikes that used it still around that it 
would make sense to make a run of them. Wonder if there is
a problem with the mold or something?

Here's a Shinko 241 3.00 X 18 on my 1971 CT-1, with the original Yokohama Trail Sport 3.25 X 18 on the left. Looks nice and rides well. The Shinko is about 1/2 - 3/4" shorter, if that matters to you. I have a Shinko 3.50 X 18 on back. Revzilla has good prices on them, and good stock.

 

I put the same on my CT3. Hope to take it on a dual sport ride next weekend.
 

Thanks everyone for the good tire info.