Recommended Tire Size for Upgrading of LT Trailer Tire with ST Trailer Tire

Question:

I have an old 19 foot ski boat with trailer that currently has 7.00R15 LT tires. Ive read a lot this week about using ST tires for trailers. They did not exist when my trailer was made. I have read many places that there is not any tire company that has an ST tire that exactly replaces the dimensions of the 7.00R15. From what I have gathered, the closest match would be an ST225/75R15. The Carlisle Radian RH seems to have the best reviews. The Carlisle RH ST225/75R15 has a diameter of 28.3. The current 7.00R15 has a diameter of 29.8. Is this an acceptable replacement for the old LT tires? Lastly, the weight of the boat and trailer with full fuel tank is about 4000 lbs. The ST tire comes in both D and E weight rating. The trailer is a single axle. Should I go with D or E? Thanks experts!

asked by: Boat M

Expert Reply:

Good news, I can help you find a tire for your Ski Boat Trailer.

I wasn't able to find the Carlisle Radian RH you referenced but, the closest size to a 7.00x15 tire that we offer would be a 225/75-15. One 225/75-15 tire that could work for you is the Kenda # AM10303, this tire would be slightly shorter and a little wider than your 7.00-15 tire. For a load range D that is slightly shorter and wider, I would recommend the Taskmaster # TTWTRTM2257515D.

Something to keep in mind is that a 7.00-15 tire typically fits 15 x 5-1/2 inch wheel. The ST225/75R15 tire size will fit on 15 inch diameter rims that are 6, 6-1/2, or 7 inches wide. They will not fit on rims narrower than 6 inches. I recommend checking the width of your rims to determine if the new tires will fit. Another thing to note when moving to a different tire size is fender and frame clearance.

A 7.00-15 tire will be about 8 inches wide and have an overall diameter of about 29 inches. The overall diameter of a 225/75-15 tire is about 28 inches and it will be about 9 inches wide. This means the new tires will sit about 1 inch closer to the inside of your fender or trailer frame. I recommend measuring the space you have available to make sure a wider tire will fit.

Typically 225/75-15 tires that have a load range of D have a capacity of 2,540lbs at 65psi and load range E have 2,961lbs at 90psi. Since your trailer is a single axle and will weight about 4,000lbs when fully loaded either will work for you, but I would recommend the Load Range D which is more comparable to your 7.00-15LT.

We do not carry a 7.00-15LT Tire. The 7.00-15LT Tire would be an exact replacement for what you currently have on your trailer. Just wanted to let you know that when the LT is at the end of the tire size then it is fine for trailer use though they are not as common anymore.

I linked a few Help Articles on trailer tire sizes for your convenience.