Consider 27x8.5 R14.  Firestone Firehawk R4S can be had for as little as $63
a pop from Tire Rack, although they don't always have them; This is the tire
I'm currently using with good results.  There are other brands available in
that size.  BTW this is roughly equivalent to a 215 section width tire.  The
circumference is specified at 26.5 inches.

Chris.