QuestionWe are trying to resurrect a 1976 Airstream that currently has 7.00/15 LT tires on it. Although we can find these with some difficulty, a less expensive offer on ST 225/75/R15 has come up. Is this truly "the same" size, or really close, as the salesman suggests?
AnswerChip,
First, there is no "same size" in switching out of a 7.00-15LT. Tires with that type of sizing designation are "sized" differently than they way it is done now.
A 7.00-15LT is 8.0" wide on a 5½" rim with a diameter of 29.6" and a load carrying capacity of 1720# @ 45 psi (Load Range C), or 2040# at 60 psi (Load Range D). It is important that you find out which Load Range these tires are. BTW, another way of expressing this is "Ply Rating".
An ST 205/75R15 is 8.0" wide on a 5½" rim with a diameter of 27.1" and a load carrying capacity of 1820# at 50 psi (Load Range C) and 2150# at 65 psi. Interestingly, the load carrying capacities for this size is the same at the pressures enumerated for the 7.00-15LT.
By contrast an ST225/75R15 is 8.8" wide on a 6" rim with a diameter of 28.3" and ...... I'm not going to write out the load because it is higher.
Aside from the diameter, the ST205/75R15 most closely matches the 7.00-15LT - and that's where I would go, being careful to match the load range and use the higher inflation pressure. Also check to see a 1¼" lower ride height is OK.
