Only as a must. But even then I would only put it on front end. The more if
you have LSD diff (not likely, but still).

http://www.miata.net/garage/tirecalc.html
It will be 0,7 % less to fast, that means when tacho says 60 mph, you are
acutally doing 59,6 :-)

--
It's a good idea to drive a little Japanese/Ford/whatever shopping
machine once a year to remind yourself why you're in a BMW for the
rest
of the year. by John Burns.
'98 328i coupe
t.

Revolutions per mile are virtually identical for those two sizes.  For
example, the Continental CH95 specs at 833 for the 205/60R15 and 835 for the
225/50R16 - a difference of about 2/10 of one percent.

http://www.tirerack.com/tires/Spec.jsp?make=Continental&model=ContiTouringContac
t+CH95

Tom K.

Thanks guys
That was a lot of help. I had imagined that a different tyre radius (ie 15
or 16) meant a different circumference, but it just seems to be a different
size hole in the middle of the tyre!

Thanks again

Yes.  They are approximately the same diameter and, therefore,
circumference.  225/50-16 wheel sets came on that car with the sport
package.

Yes.  Same circumference means the speedo reads the same.

The 205/60x15 and the 225/50x16 are functional equivelents. The speedometer
won't care. The difference in the overall diameter is less than 0.2 inches.

Yes, the 15 was more than likely the standard tire, somebody decided to
upgrade to 16s. The "right" way to upgrade would have been to buy 5 new
tires and wheels, that's what I did when I upgraded my E36 to 17s.