Thanks for the detailed response Peter. I find the last paragraph confusing when you say that the 205 55 15 is just too small because you say the 205 width is ok but the 55 is too small although the diameter is 23.88 compared to 23.86 for the 225 50 15. Tire.
Firstly, you're likely looking at & comparing the
NOMINAL tire sizes rather the
ACTUAL PHYSICAL DIMENSIONS of the tires, and they can differ
A LOT! 
Secondly, you also need to remember that the OEM 225/50R15 is
TOO SMALL of a ROLLING DIAMETER and is TOO WIDE for your Spyder, the
TOO SMALL bit giving many seriously optimistic Speedo's, trip meter's, & odometers - so you might
THINK you're doing saaaayy, 60mph and travelling 60 miles in each hour you ride at that speed,
but you're not, you're likely really only doing about something like 54-55 mph or so, maybe a tad less or more, and you probably only travelled around 55 miles or so, give or take maybe 5 miles - but it could be that you were only doing 53mph and you travelled 58 mph, cos each bike and each tire and each tire pressure and each speedo and each odometer and each ambient temperature & road temperature is likely to be slightly different, so while things might be close to your buddy's bike or someone else's on the Forum, it's unlikely to be the same,
but in factory trim on factory OEM Tires, they'll ALL BE OPTIMISTIC to some (variable) degree!!
Next, there's the
TOO WIDE bit - the OEM width tire,
AND many replacement tires of the same Nominal width are
simply too wide for the OEM rim, it makes the tread (that should be flat to sit flat on the road surface) bulge up in the middle because the beads are pinched too closely together when you fit those tires onto the rim, meaning that you
WILL be running on a narrow '
less than full tread width' strip in the middle of the tread, even on a heavier/stronger built auto tire (altho it might be a tad wider than that of the OEM) while the OEM Tires make this '
only run on & wear the strip in the middle' thing even worse by having such a light construction that the tread plies
CAN'T hold the tread even slightly flat on the road surface once the tire starts rotating at speed, throwing/bulging the centre of the tread
even further out and narrowing the little strip of rubber that you're actually running/riding on even more!!
So while running a
SMALL DIA tire isn't really such a biggie, it isn't helping you any, altho it
might be fooling you into thinking you're riding faster and getting better fuel economy that you really are. But running a
TOO WIDE tire for the Rim
IS reducing your contact patch; making the centre of the tread wear more/quicker than it needs to; exacerbating lack of traction issues and increasing your chances of hydroplaning in the wet and scoring tire damage if you hit road debris; and a bunch of other stuff that's not really good for you/your safety/your pocket, et al. :banghead:
How can I put all this simply.... OK - You can fix this by running a narrower tire, saaay, a 215 or a 205,
BUT, if you reduce the width of the tread, the 225, 215, or 205 bit,
YOU NEED to increase the Profile accordingly, or
YOU NEGATE THE IMPROVEMENTS OR YOU MAKE ALL THOSE BAD THINGS WORSE!! Go
down one size in width, go
up one size in profile is the 'standard practice' in order to maintain the designed dynamics of the machine, and that means that if you
go down TWO sizes in width, like from a 225 to a 205, you really need to
go UP TWO sizes in profile, ie from 50 to 60
AT LEAST, or you are just repeating &/or compounding all those issues identified above instead of improving/reducing them, which is generally at least
part of the reason for changing the tire to something other than the OEM Kenda, isn't it?? :dontknow:
If you want better tire life, better tread wear, better ride, better road holding, better traction, better handling, better ability to withstand road debris strikes without damage, a closer to accurate speedo & odo &/or any/all those other things, mentioned or glossed over above, then running a 215/55R15
OR a 205/60R15,
maybe even a 215/60R15 or a 205/65R15 will do that, but running a 205/55R15 is pretty much a no gainer, cos it's
TOO SMALL of a rolling diameter, just like the OEM Kendas are,
but it's NOT TOO NARROW, for all the points raised above. Make sense?? :dontknow:
Ps: Normal tire sizing/naming conventions call for the inclusion of the 215
/65
R15 bits between the numbers, cos without them,
205 65 15 might be just numbers, but
with them,
205/65R15 IS an internationally recognized tire size. :thumbup:
) auto 205 wide tire, because the 225 wide tires on the OEM rim ALREADY tend to pinch the sidewalls in a little, making the centre of the tread bulge a bit even before it gets thrown out & bulges even more when it starts turning under power!! :gaah: The narrower 215 or 205 tires with stronger/heavier tread plies won't do that anywhere near so much, if at all! 
hyea: 
