I've heard 15" tires are better than 14" tires. How big of a
difference is there? Is there much of a safety benefit to 15" tires?

Dear Gremlin, 

Most people seem to think that in tires, size matters. 15'' tires are
better than 14'' ones, since they will give you better handling, more
horse power and you'll be more stable - because your centre of gravity
is lower.

Because they are heavier, this could also bring about an increase in
fuel consumption; they are also expensive; and the rear storage space
is lost to a bigger spare.

However, do not overestimate its effect - upgrading from 14'' to 15''
is not such a huge change that would revolutionise your driving, and
upgrading more, might not be much of an upgrade, regarding weight and
fuel consumption.

There are fringe benefits - according to an article on the subject, it
also gives your car a cool look.

Earle Eldridge "Move to bigger tires could cost drivers", USA Today,
March 6, 2003 - http://www.thetowntalk.com/html/B3B671EB-7DD7-40F4-A93C-71F8AB57AB3E.shtml,
mostly against the idea, but mentions also its advantages.

New York Fan, Is it legal and good to change
tires?<http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&threadm=6ssr9t%24cu%241%40winter.news.erols.com&rnum=1&prev=/groups%3Fhl%3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26selm%3D6ssr9t%2524cu%25241%2540winter.news.erols.com>
- A Honda discussion, on top of it :-)

Micha, 14 inch tires vs. 15 inch ones,
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&threadm=Cjim6.841%24H22.10632%40news1.mts.net&rnum=1&prev=/groups%3Fhl%3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26selm%3DCjim6.841%2524H22.10632%2540news1.mts.net
 - another Honda related discussion.

Synapsyxy, 14" to 15" tires,
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&threadm=19990809212801.10818.00003796%40ng-bd1.aol.com&rnum=1&prev=/groups%3Fhl%3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26selm%3D19990809212801.10818.00003796%2540ng-bd1.aol.com
- a discussion on the technical considerations.

Search terms: 
"15 inch" "14 inch" tires 
"15 inch tires" "14 inch tires"
"15" tires" "14" tires" 

I hope this answered your question. Please contact me if you need any
clarification on my answer. I'd be pleased to clarify my answer before
you rate it.

"Most people seem to think that in tires, size matters. 15'' tires are
better than 14'' ones, since they will give you better handling, more
horse power and you'll be more stable - because your centre of gravity
is lower."

- the shorter sidewall will give better handling if tires are exactly
the same between the 14" and 15"
- there is no correlation between horsepower and wheel weight - the
lighter the wheel/tire combination, the less hp is lost because the
engine does not have to work to push as much weight, but there is no
hp change.
- you won't have a lower center of gravity if you adhere to +1 (or
+2,3,etc.) sizing. as the rim size increases, the tire sidewall
decreases, thus there is almost no measurable change in ride height.

"Because they are heavier, this could also bring about an increase in
fuel consumption; they are also expensive; and the rear storage space
is lost to a bigger spare."
- chances are, if someone upgrades to 15 inch wheels, they will be
upgrading from steel to aluminum wheels. at that point they will
proabably drop 3-4 lbs per wheel in weight, and the tire weight should
stay approximately the same because they have a lower profile tire
that is now wider.
- why would you change the spare if the overall diameter of the tires
has not changed?

i think your answer was very misguiding and actually wrong. besides
the fact that your ideas on the subject are wrong, you need to read
the original question again and you will see that you completely
missed the question - is there much of a safety benefit to 15" tires?

- there is if you value shorter braking distances (due to the larger
contact patch) and if better response is desired in emergency
situations (ie. emergency lane changes)

Is there any data out there on whether the shorter braking distance
and better handling lead to a reduction in accidents?

I'm doing this as a comment rather than a clarification request
because my original question has been adequately answered.

I agree with always on some points. The horse power does not change,
and would not change even if you went to a larger tire diameter.
Larger tire diameter would reduce the torque and perhaps give very
slightly more miles per gallon. I would think the cost of changing,
off sets any advantages in most situations. I got the impression that
a wider rim is practical with the 15 inch wheel, but I have no idea
what rim widths are available. With some combinations the space
requied in the trunk would be about the same.   Neil