Tires: Tyre size and rim compatability, tyre width, tyre size

Question
QUESTION: I have 520 x 14 crossply tyres fitted to a 3.00 x 14" rims (Morris Minor). The use is for a very light weight, domestic trailer carrying 5cwt max.  I realise I will need to change to radials. Can I presume I can fit a 145 x 14 or a 155 x 14 but can I fit a 175 x 14 which I have access to?

ANSWER: Brian,

There is literally nothing that will properly fit the rim you have.  That leaves you with 2 options:  Change the rim or select a tire knowing that it isn't a proper fitment.  I would go with the latter - again, knowing the rim is too narrow.  Since this is a trailer and you'll only be using it for short distances, I don't thing the problems associated with narrow rims - wear and directional stability - are larger enough to worry about.  But it is important to consider load carrying capacity.  

A 5.20-14 came in 2 Ply Ratings and those had different load carrying capacities:  4 PR:  375kg @ 2.1 bar and 6 PR: 425 kg @ 2.8 bar.  The other bit of info that you need is that 375kg = 74 Load Index and 425 kg = 78 LI.  You'll want to verify that the info I just gave you is what is on the sidewall of the tire.  If not, then what I am about to write will be incorrect - and you should ignore that info and post a followup.

Assuming what I wrote is correct, you need to select a passenger type tire with at least a Load Index equal to what you have.  A 145/80R14 has a 76 LI and a 155/80R14 has a 81 LI.  So that should help you decide what to do.



---------- FOLLOW-UP ----------

QUESTION: If I have understood the details of your answer, to replace original 520 x 14 with a 145/80R14 will offer a similar LI i.e. 76 rather than 74  (The previous tyre was a 4 ply).  Am I correct in thinking that the higher the LI the better?  If so will a 155/80R14 be better than a 145/80R14.

I assume from your comments that the tyre width is of less importance, but what's the limit for the rims? You suggest that 155/80R14 will fit but what would the limit be? 175?

I am unlikely to exceed 50  mph and load of 375kg.

Brian

Answer
Brian,

I'm glad you clarified the issue of the PR.  And just to point out, the 375 kg load is load on the trailer plus the weight of the trailer itself - times 2 because there are 2 tires.

Would a 155/80R14 be better - well, yes, larger LI's help durability - except that even further aggravates the narrow rim issue.  On the other hand, we know we are not following the rim recommendation and don't care about the wear, so .........