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Old 06-28-2006, 10:41 PM   #1
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1965 26' Overlander
Tulsa , Oklahoma
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Tires (again)

Can I put 225/75/15's in place of my 7.00/15's on my 1963 19' GT?
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Old 06-28-2006, 11:39 PM   #2
Creampuff
 
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1957 22' Flying Cloud
1971 31' Sovereign
1976 29' Ambassador
Malibu , California
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Short answer IMHO- Yes! I have changed up on several trailers, but be aware you may have a problem getting the extra width over the top edge of the hub as you install. The aging torsion axles tend to have lost their 22 1/2 degree down- angle which affects your wheel well clearance, so sometimes I've had to deflate the tire to get it on. Otherwise, as long as you can get it on the hub ,the running clearance is fine.
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Old 06-29-2006, 01:11 PM   #3
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1978 24' Argosy 24
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Does anyone make an ST215/75R15 ? They make the 215 for 14" rims and since the majority of old 15" tires were 15x7.00 it seems the 215, being closer in diameter and section width would be the natural size to produce. It's amazing that all of the competition seems to want to go head to head with the exact same tire offerings while leaving this obvious void. Very irritating!
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Old 06-29-2006, 01:42 PM   #4
Frank S
 
1973 27' Overlander
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Hi bhayden--Don't think you want ST215/75R15 as if they made one, the diameter would be 2.1 inches smaller than 7.00-15. ST225/75R15 is the radial replacement for 7.00-15, and it is also smaller in diameter by 1.5 inches. The 7.00-15 has a diameter of 29.8 inches. My experience has been that a vintage A/S naturally gets lower to the ground with time, so I've stayed with 7.00-15 LT.--Frank S
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Old 06-29-2006, 04:10 PM   #5
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1978 24' Argosy 24
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Thanks for the heads up! I had a hard time figuring out what size the original tires were supposed to be back in 1978. I know it's spec'd to have 7.00x15 tires and that's what's on the trailer now but they're an off brand and from what I could tell 7.00x15 evolved to mean different things over the years. From what I was able to determine the sizes ending in .00 had different aspect rations than those ending in .50 (as well as different section widths of course). My "best guess" for 7.00x15 tires in the mid '70s was an aspect ration of 94%. I assumed the section width was 7", I'm not at all sure that either of those assumptions is true.

Here is an interesting document with loads of info on tire sizes and load vs inflation:

http://www.goodyear.com/truck/pdf/engdatabook.pdf

I noted that Goodyear still has listed a 7.00x15 Workhorse Rib (pg . There's some interesting data here. First off the designed rim width is 5.50"; I thought it was 6". I'll have to be sure to have the rims measured when I get the tires replace (yeah, I could do it with the tires on). I was surprised that the section width is 8.0" inflated and 8.6" loaded. Radials like the Marathon bulge almost twice as much. Another surprise was the difference between "Inflated Dimensions" and "Loaded Dimensions". This isn't usually spec'd in the manufacturers data. For the 7.00x15 Workhorse it's 30.0" inflated but the Static Loaded Radius is only 13.8". Somewhere they must spec the inflation PSI and load but I don't see it right off. Using these numbers you'd come up with a tire height of 28.8" when it's on the trailer which is about what I measured.

The data for the ST Marathons is on page 59. Designed rim width for the 205/75R15 is 5.50 and for the 225/75R15 it's 6.00". The static loaded radius for the 225 is only 13.0" so the trailer sits almost an inch short even though the tire is an inch wider . At least they weight the same 27#

Now, looking at my "beloved" Wrangler HT (pg 54 for those reading along at home ). First off they skip from 215 to 235 (strange as 225 is the more common size). The 235 might be out because of clearance problems with width and because the designed rim width jumps up to 6.5; have to look at the chart for "acceptable" rim widths and measure mine. The LT tires are load range D which from what I've read equates to an "8 ply" rating yet has roughly the same load rating as a Special Trailer (ST) tire of Load Range C. Most LT tires are spec'd only with the newer "service description" which for this size Wrangler is 106/103Q (Q being a speed rating). Although the Wrangle LT215/75R15 is shorter (27.7" inflated vs. 28.3") it's Static Loaded Radius is identical to the 225 Marathon (13.0") and for some odd reason the Gross Contact Area is greater (39sqin vs 36sqin or about 8%). Load range is comparable (2150 vs 2095) with the caveat that the Wrangler is spec'd at 65psi and the Marathon at 50psi.

There's issues with width (fender well clearance and rim width) but an LT235/75R15 certainly opens up a lot more possibilities. With the Wrangle HT the 235/75 is still shorter but much closer to the original. You only lose half the ground clearance of (13.4" loaded radius) compared to going with the ST225 (13.0" loaded radius). Unfortunately with this tire you seem to loose load capacity going to the bigger tire which is strange?

Lots to consider. Good thing I'm starting my search for winter tires in June ! One thing that's becoming a familiar refrain is the ST225/75R15 keeps coming around on the guitar.

-Bernie
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Old 06-29-2006, 06:45 PM   #6
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Hi bhayden--Thanks for the great info on the Goodyear Engineering Data Book. I downloaded the whole thing (more info than I'll ever need, but I love tire stuff) and can have some fun playing with the info. Never know too much about tires.--Frank S
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Old 06-29-2006, 09:58 PM   #7
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1957 22' Flying Cloud
1971 31' Sovereign
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Workhorse Rib

I bought a pair of these for my first '67 Safari and nearly went balistic when the guy had to put 8 oz. of weights on each to get them balanced. I asked about it and he first blamed Venezula ( I think) where they're made. Then he blamed my original rims, which admittedly were a tiny bit wonky. I nievely paid $115/ ea special ordered (then he dinged me for the balancing- by the pound!) for these donuts, wanting to stay 'original"- not knowing that Marathons were cheaper, better and that the wheel really had been re-invented as a radial.
They're in storage off the rims now - 3 trailers later, and I just put a set of Marathons on the '57 FC- for what all that's worth!!
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