tire help - '65 c10 - 15 inch wheel

[Woogeroo]

That is the wheel on my truck, I have been told that it is the original type wheel for 1965.

The size of the tires currently on there:

Tire Size = P235/75R15

Which is a passenger car tire which has a lower load rating... and since I actually use my truck as a truck now and then - I want the LT - light truck tires... also, it turns out the tires that are on it has a rolling radius of 28-29 inches... but the original tire size was something like a 31 inch rolling radius... but I dunno...

I have no idea what tires to get, I'm so confuzzled.

If I drive 10 miles on the interstate by the mile markers, my odometer shows that I went 8.5 miles... I am thinking this is partly to do with the smaller rolling radius of the tires that are on it.

According to my 1965 shop manual:

C15(127 inch wheelbase)

Fronts:

7.10-15-6

7-17.5.6


Rears:

7.10-15-6

7-17.5.6

same as above... but uh, that is the 'olde' way of tire sizes... right? How to find out or figure out which size came on my truck?


so, if any of you guys or gals know anything about tires or the original tires for the 15 inch wheel - and the modern equivalent, please let me know. I want to drive my truck again.



Thanks,

Very ignorant about tires...

-W