Ok, for everyone who comes across this thread in the future, I have found a
site which I believe helps out. The address is
http://www.loadrite.com/supportcente...ral_spec3.html

It would appear the metric tire to replace an A78-13 tire would be
ST165/80B13



"
Hi there,

Just wondering if any of you have replaced your old boat trailer tires
recently, and that would be in Canada I suppose. The tires on my old
trailer state A78-13 as the size. Do you think I can find these in the
stores? No way. Us Canadians have to have our own measuring scheme and
then refuse to build a calculator to show the comparison of metric to
imperial.

Does anyone know what size of metric trailer tire would be the equal for
this old A78-13 tire? I've had no success in finding an online calculator
or conversion chart.

Thanks
Chris