Hi, and welcome to the forum, that 's a great deal on the trailer, way to go!!
Car tires are perfectly fine if the tire selected meets the trailer load requirements, I plan to do this in the future, I have selected the "General Altimax RT" radial in a 205/70R14 size, here are the specs for this tire:
General Altimax RT 205/70R14 98T XL
Load Index 98 = 1653lbs (750kg) per tire
Speed Rating “T” = 118mph (190kph)
UTQG: A600 A A
Load Weight per Tire = 1653 lbs.
Tire Pressure = 51 psi
Tread Depth = 11/32"
Tire Weight = 21 lbs.
Rim Width Range = 5-7"
Measured Rim Width = 6"
Section Width = 8.2"
Overall Diameter = 25.3"
Revolutions per Mile = 821
Now compare this to the Goodyear Marathon ST205/75R14 specs:
ST205/75R14 Load Range C Trailer Use Only
Max Load per Tire = 1760 lbs.
Max Tire Pressure = 50 psi
Max Speed Rating = 65mph
Tread Depth = 10/32"
Tire Weight = 23 lbs.
Rim Width Range = 5-6.5"
Measured Rim Width = 5.5"
Section Width = 8"
Tread Width = 6"
Overall Diameter = 26.1"
Revolutions per Mile = 798
My trailer's gross weight is 6040lbs and these tires have a max load of 1653lbs per tire: 1653 X 4 = 6612 lbs (more than the gross weight of my trailer), the tire diameter difference is only 3/4 of an inch so the trailer will sit 3/4 of an inch lower than with the Marathons. These tires ARE NOT MADE IN CHINA so you have a better chance of a quality made tire. My Dexter axle manual that came with my trailer says on page 70 that ST, LT, P-Metric, and E-Metric tires all can be used. The only restriction mentioned is that if Passenger tires are used, they must be derated by 10%, so if we take 6612 lbs and multiply this number by 90% we have 5950.8 lbs (still acceptable since I tow at a weight of 5600lbs) and you have to subtract the tongue weight from the gross weight to get the axle load (5600 - 500 = 5100lbs) and I don't know if this even applies to this tire since it's an E-Metric tire and not a P-Metric, and they will fit on my existing rims that came with the trailer.
I might add that the 1969 Avion that I once owned came with car tires on it from the factory.
2005 Springdale 249BH FW
2008 Toyota Tundra Crewmax 5.7L