What size are the 13's,  what size are the 14's?    If the circumference is within 5% or less I wouldn't worry about using the 13 for a spare.   

 

 

Also, maybe goodyear trailer tires are better than they used to be.  I had nothing but trouble with them on my race car trailers.   

You are thinking about the tire differences in reference to mechanical breakdown, which is a good idea and, as many have said, a short distance at low speeds you'd certainly be ok.  However, you are asking about a hundred miles.  Does that mean on an interstate or at 55+ MPH?  If that's the case, you go from "ok" to "a danger to yourself and others".  I have pulled quite a few things of sizes ranging from tiny to I-shouldn't-be-towing-this.  I feel you are under the impression that the light weight of your boat and trailer combination is safer than a heavier combo in this situation and, if so, you could not be more wrong. 

 

A heavy load/trailer is going to give you a significant amount of "input" to your tow vehicle and, thus, probably warn you in advance of an impending emergency.  Your light boat/trailer combo with mis-sized tires will NOT give you warning.  A change of road surface or road pitch (left to right) at highway speed and your light boat and trailer will be directly next to you and facing the incorrect direction. 

 

If you are talking about limping your boat home a few miles at 35mph I've got no problem with that.  If you are talking about 55mph or higher please do not take this chance. 

Fun fact: in the year since I bought them the 14” Goodyear Endurance tires have increased from $118 to $160.

Just now, Elkins45 said:

Fun fact: in the year since I bought them the 14” Goodyear Endurance tires have increased from $118 to $160.

Lol not sure I'd call that a "fun fact."  More like a four letter swear word.

4 hours ago, Woody B said:

What size are the 13's,  what size are the 14's?    If the circumference is within 5% or less I wouldn't worry about using the 13 for a spare.   

 

 

Also, maybe goodyear trailer tires are better than they used to be.  I had nothing but trouble with them on my race car trailers.   

175/80/13 vs 205/75/14 so 75” circumference vs 80”. 
 

24” vs 28” diameter.

 

If it is for a spare that you won't likely have to use, I would get a cheap one. I see rim and tire in your size for as little as $70. It's most likely not going to be needed and and only going to rot in the sun, but if you need it, you've got it.

Look at your tire size, rating and bolt pattern, get on Amazon and order..... it's that simple.

I paid $68 for tire and rim.

6 hours ago, Big Hands said:

and only going to rot in the sun

Why is it going to rot in the sun?  Mine is stored in the garage 90% of the time.

I don’t have a garage 

I have a garage, but it's such a pain getting the boat in and out of it, I just use it for winter storage...rest of the time the boat sits next to the garage.

2 hours ago, gimruis said:

Why is it going to rot in the sun?  Mine is stored in the garage 90% of the time.

The ozone in the air will also rot a tire, just like the UV light from the sun.  It'll last longer if kept out of the sun, but it'll still rot.  

 

Fun fact:   How many rubber molecules are in a tire?  One!  All of that rubber forms a single, gigantic, polymer molecule.    

6 hours ago, Bankc said:

It'll last longer if kept out of the sun, but it'll still rot.  

I put new trailer tires on my boat in June 2022.  When I had the tire shop change out the rims from the old tires and put the new tires on, they also balanced them.  I also had them look at and visually inspect the spare, and they said it was still perfectly fine.