QuestionWhat is the best tire to get for my horse trailer that has 6.70x15LT tires on it. Want new tires and all I can find are tires that are vintage in this size. What is best in size to replace. Any help you can be will be appreciated. I have an old Hale thoroughbred double horse trailer.
AnswerLionna,
A 6.70-15LT is 28.4" in diameter, 7.5" wide on a 5.0" rim, with an allowable rim width range of 4.5" to 6.0", and with a load carrying capacity of 1520# (load Range C = 6 Ply Rating), 1610# (Load Range D = 8 Ply Rating), or 1820# (Load Range E = 10 Ply Rating)
An ST205/75R15 is 27.1" in diameter, 8.0" wide on a 5.5" rim (7.8" wide on a 5.0" rim) with an allowable rim width range of 5.0" to 6.0", and with a load carrying capacity of 1820# (Load Range C) and 2150# (load Range E).
This looks like at good fit with 2 exceptions: If you have a 4.5" rim width (which should be stamped somewhere on the wheel, usually behind the backing plate), and this doesn't work if the current tire is a Load Range E = 10 Ply Rating.
- BUT -
I am of the opinion that trailers are frequently under tired, so if it is possible I would go to the larger ST size - ST225/75R15 - and get the largest load range you can find and use the max pressure.
An ST225/75R15 is 28.4" in diameter (that doesn't look like an issue), 8.8" wide on a 6.0" wheel, (this might be an issue as on a 5.0" wheel - which it isn't supposed to be on - the width is still 8.4" wide, so you need 1/4" more clearance on each side and the clearance when the vehicle is in motion is really difficult to measure. Note the tire flexes and distorts and you can't really measure it statically. ) But you gain a whole bunch of load carrying capacity (The tire is rated at 2540# for a Load Range D and 2830# for a Load Range E.)
So here is what I would do:
1) If you can weigh the trailer fully loaded before purchasing tires, that would be good. If you can weigh each wheel position individually, great. (There are procedures on the internet somewhere to help you get the individual wheel position weights). If not, weigh the trailer axle by axle, if you can - than add 10% to the individual loading.) - or if you can't do that, weigh the tire loads for both axles and add 15%.
What you're trying to do is find the worst case.
Then to the worst case add another 15%.
That should be the minimum load carrying capacity for an ST (which will restrict you to 65 mph). If you need to go faster, then you need to add another 20% to the load carrying capacity to get you to 85 mph.
2) Then the clearance - It doesn't look like you're going to have diameter issues, but the width is another kettle of fish. I suggest you hook up the trailer and back it (slowly) on a paved surface so that the trailer jack-knifes. You should see the tires bend inwards and outwards. Park the truck and measure the clearance. I'm not sure how much more you need than that, but if you are anywhere close, you should be concerned.
OK, that was a lot of complication, but I'm trying to avoid problems ahead of time.
Hope this helps, and post back if you need additional help.
Barry
