I run BF Goodrich TAs 215 65 R15 on my 91 Carat Vanagon with no problems.  I
really like this tire as it has excellent street characteristics.  I have
also checked the speedometer and odometer with mile markers while driving and
found that they are both right on the money.  This was over the course of a
several hundred mile trip.  This would make me think that running these tires
with the 15" rims is actually giving you the same or very similar rolling
dimensions as the 14" rims with 185 75 R14 (that is why the speedo is still
correct).  If this is true then how could this adversely effect gearing?  I
haven't noticed any less power with my van than I had before with the 14"
tires and I have noticed quite a nicer ride due to the nicer tires I could
afford to put on these rims (BF Goodrich is a great quality tire, but cheaper
than Michelin).  I have been running this way for about a year now.  Basing
my statements on what I have experienced.  That's all I can do.  I am not a
tire expert, and don't pretend to be.

Thanks,
Ken Wilford
John 3:16
www.vanagain.com
Phone: (856)-765-1583
Fax: (856)-327-2242