B210 sleeper
I have just hit the books & it looks like the195x50x15 tyres that you are using are a nominal 577mm in overall diameter, while my B310 owners manual says the original size was 155-SR-13, which measures a nominal 580mm
Several options present themselves
1. Look for an overdrive five speed box [if you dont already have one.]
2. Find a 3.54 to 1 diff
3. Use larger diameter tyres on the rear.
205x50x15 = 587mm.
215x50x15 = 597mm. Even this can be used on a rim as narrow as 5-1/2 inches.

A combination of these should give you more "gear" than you would ever need.