If the car has much more than 15,000 original miles on it, I very
much doubt any of the tires are original. The only way an
original tire would be left if it was the spare and never used.
It happens, but not very often. If the rims show alot of road
use, it would also show that the tires may not be original.
The 5.50/5.60 and 5.90 designation have to do with the tread
width in inches of the tire. Because of the different method of
making bias ply tires the 5.90 was slightly bigger around than
the 5.50/5.60.
Besides 6 ply tires being stiffer, they also have a larger load
rating. You have to be carefull when reading ply rating. Some 6
ply tires do not have six plys: the 6 ply indicating it can carry
the 6 ply load using only four plys. I think six ply is way to
heavy a tire for a light car like the MG. It was more intended
for trucks.
However, bias ply tires are much stiffer than radials. When I
first got my TD, I put 4 ply bias tires on it. With higher tire
pressures, 26 psi, it rides like a spring wagon, and I've ridden
alot on a spring wagon. It also tended to wander around and skip
and slide at every groove like a train wobbles on the tracks.
While not original, radials will give a better ride and handling.
If you plan on doing much distance driving, carefully consider
the radial option of 165SR75X15 or the 165SR70X15. 165=tread
width in mm, S=standard speed rated, R=radial, 75=aspect ratio
(75% as high as the maximum overall width), X="by", and 15=rim
diameter in inches. The 75 aspect ratio gives a little larger
diameter tire than the 70 aspect ratio, and it will lower the
engine revs per mile some. Some USA TD's did come with white
walls as I've seen pictures and comments. But, they are the
1950's style WIDE stripe. My radials are mounted with the white
wall in.
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