tire sizes

Thanks Semo

No, they are not the same height. But they are not far off. There are a number of variables that need to be considered however. Most manufacturers publish dimensional characteristics of their tires. Look for a dimension called "static loaded radius". This would be the dimension from the ground to the axle centerline. Use the same manufacturer, ply and tread design. Here's an example, using Firestone:

Tire - Traction Field & Road - R-1 Tread, 6 ply
18.4x28 on 16" wide rim, 16 psig inflation, 3960# load = 26.7" static loaded radius
15.5x38 on 14" wide rim, 20 psig inflation, 3520# load = 28.4" static loaded radius

Conclusion: a comparison of the two tire sizes, each under their own unique rated conditions, would indicate that the 18.4x28 tire/rim combo would result in the axle sitting 1.7" lower than the 15.5x38 tire/rim combo. A change in any of the above variables can result in a different conclusion, although I wouldn't expect minor variations would result in a significantly different result.

Rod

There is no chart that I am aware of of tire height. Different manufacturers actually don't make their tires the same height. Good Year, for one, had tires that were noticeably lower with the same designation as Firestone. Theoretically, the first number refers to the diameter of the cross section of the tire. Approximate height is calculated by taking twice this diameter plus the second number which is the rim diameter. The resulting number needs to be reduced by the amount that the tire circle is cut by the rim. Usually, this is a couple of inches per side of the rim (2). But the amount varies with the rim width and the tire width.

In your case, the 15.5-38 would be about 4" taller than the 18.4-28 if the rims were matched to the tire size. If the rim was a 15" on the 15.5-38 and 12" or 14" on the 18.4-28, the height of them would be similar. I estimate the height of your 15.5 tires to be 64". The 18.4 tires on 28 rims would be about 60"

The big issue is that they are not interchangeable. If you change the rim as well, the hubs may be different unless both wheels come from 180 tractors. And even then, they may be different.

Thanks