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

Question

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

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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.

expert reply by: Jackie C

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Answer 1

It's pretty close in size. The ST225/75-15 will typically be about 28-1/2 inches in diameter by about 9 inches wide, while the 7.00-15LT tire will be about an inch larger in diameter at about 29-1/2 inches and an inch narrower at about 8…

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Answer 2

Yes, the Karrier Trailer Tire Size ST225/75R15 #AM10303 will fit the wheels you have that currently have tire size 7.00-15LT on them.

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Answer 3

There is not an equivalent trailer tire to your light truck tires that are 7.00-15, which have an overall diameter of 29.4" and width of 7.8". A 225/75-15 tire such as the Karrier #AM10256 is similar, with a 28.4" diameter…

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Answer 4

The 7.00-15 tires you have are not exactly the same size as an ST225/75D15 tire. The 225/75 is going to be about 1/4 inch wider and a little over an inch shorter. So #AM3S870 would work as a temporary spare but not in the long term. You would want to…

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Answer 5

Good news, I can help you find a tire for your Sundown Horse Trailer. The 205/90-15 is a close tire size to the 7.00-15. 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…

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Answer 6

You will first want to check the fuses on your tow vehicle. Some vehicles could have fuses for operating the turn signals and brake lights of a towing package. You will want to check if they are installed and working properly. The next thing is to…

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Answer 7

Typically, the black wire is 12-volt power, and the red wire is left turn/brake lights. But you cannot always go by color. If the wiring in question is on the trailer side, then there are two ways to determine which color wire does what function. You…

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Answer 8

It could be a tow package fuse on the vehicle. Use the owners manual to find the locations of fuses/relays related to the tow package wiring to check them and replace as needed. If the issue is not a fuse or relay then you can test the vehicle side…

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Answer 9

It is likely that the red wire on your old lights is the turn signal/brake light wire, and the black wire is the running light wire. That would make the white wire the ground. You can verify this by testing the wires using a circuit tester such as…

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Answer 10

The red wire is the turn signal wire and will be run to the green wire of your harness if its the right turn signal side, and the black wire will run to the brown wire of your harness. The yellow wire from your harness will run to the left turn…

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Answer 11

Because the size of both tires is so similar, you can certainly use the #AM31952 as a replacement. An ST185/80-13 tire will typically be slightly larger in diameter and width than an ST175/80-13 tire, but only by around 1/4 inch. Although the…

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Answer 12

If you measure the diameter of the axle you can usually determine the capacity as that's mostly standardized. Typical Axle Diameters Based on Axle Capacity - 1,000-lb - 2,000-lb Axles: 1-1/2 inch - 1-3/4 inch diameter - 3,500-lb Axles: 2-3/8 inch…

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