I went thru the trouble of measuring my wheels to see exactly what I have and the numbers seam to work out. I have an old set of RA08 185 R14 tires. I measured the circumference of one with the tire off the vehicle. The circumference is 78-7/8 inches. My newer tires,nokian hakkas 195-70R15 measure exactly the same as the stock ones. Both sets have about the same amount of wear. 
  I then did the weight on thing. I put tape on the tire and the floor, marked the tire and the floor. I then rolled the van back until the tire made one revolution and marked the floor. I then measured the distance between the marks. with the weight of the vehicle on the tires it measured 77-1/4 inches.  As you can see the weighted vehicle changed the circumference by 1-5/8 inches.
   I then did the math for RPM (revs per mile) Unweighted 803 RPMs weighted 820 RPMs  This explains my speedo error of 5 mph. My speedo says 805 is the correct number. With the weighted vehicle being 820 and 805 being the correct number. my tires are off by 15 RPMs, thus 5 mph. I also think that the standard van with RA08 tires would probably be the right tire but when you add the weight of all the camping gear it changes the circumference and the tire. 
   You can't really get the right circumference until you weight the vehicle. So, buying tires of a certain RPM is only a crap shoot. When I buy tires in the near future, I'm going to try a tire a little bigger 205 70R 15. When I buy them I'll do the same measurement I just did and post the results.  I hope this makes this perfectly clear, ha ha ha ha.