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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