Brian, it wasn't a typo. You could realize as much as that depending on the stock size and what you put on for the new size. Sidewall height also grows as a percentage of wheel diameter and overall tire height.
This is where I think the math is going wrong. The sidewall height is a percentage (given as the aspect ratio, such as "75") of the section width
, not of any diameter.
From the Goodyear Marathon
ST185/80R13 - outside diameter 24.7" - static loaded radius 10.8"
ST205/75R14 - outside diameter 26.1" - static loaded radius 11.6" (their smallest 14" tire)
For the 13" tire, my calculated diameter is
185 mm x 0.8 x 2 for two sidewalls (296 mm or 11.6")
+ 13" for the wheel
= 24.6" overall
For the 14" tire, my calculated diameter is
205 mm x 0.75 x 2 for two sidewalls ( 308 mm or 12.1")
+ 14" for the wheel
= 26.1" overall
Static loaded radius means the distance from the tire/axle centre to the ground, while carrying weight
, while not moving. This is really the value to compare for ground clearance, but the overall diameter matters for fenderwell clearance.
So it looks to me like the formula works fine, but the compression of the tire under load means that the axle is significantly closer to the ground than just half the diameter. If I compare two tires which are really the same width and aspect ratio, then only diameter difference is due to the wheel size: this is how I get that a 14" tire would raise the trailer only 1/2" compared to a 13" tire which is otherwise similar.
... any tire store can figure out the tallest possible tire combination that will fit in place of the stock 13" tire.
This is a good point - the formulas give nominal values, and every manufacturer publishes tables of actual dimensions. I like to use the formula to find a suitable candidate size (not just in trailer tires) and confirm dimensions with the tables.
Goodyear doesn't make any 14" Marathon as narrow as even the widest 13" Marathon, and all 13" sizes are 80-series while all 14" sizes are 75-series, so the closest I can see is to compare those two in my example, above. The 14" tire is a bit lower-profile (5 percentage points), but also a bit wider (about 10% wider) - the 14" tire would sit the trailer less than an inch higher, mostly due to the wheel (not the sidewall).
Of course, if there is room to go wider, a bit more can be gained: a huge ST225/75R15 would be 1.8" taller than the ST185/80R13. It would also have enough capacity to carry the entire Scamp
on one tire (at 65 PSI) - there's an idea: a unicycle trailer!
More realistically, starting with a narrow 75-series 13" tire and going to a wider 80-series 14" tire (increasing width and
aspect ratio and
wheel size) would make more difference - I guess that's what Roger is getting at.
By the way, I checked Scamp's web site, and don't see an actual tire size - I'm curious what they use.