While it is true that a larger overall tire diameter will reduce the rotational speed (for the same road speed), it will also increase the leverage of the tire when under later load (when being pushed from the side such as when turning) which is harder on the bearings.
I wouldn't worry about adequately maintained bearings surviving at highway speeds for the entire multi-decade life of the trailer. Trailers get towed much less per year than cars get driven, and similar bearings in cars last for the life of the car. I think trailer bearings are killed by their owners, rather than die from overwork spinning too fast or too long.
If the axle were replaced with the size that normally used 5-bolt hubs (3500 pound axle capacity), those hubs would use larger bearings which would have more life even without a change in tires. I wouldn't consider doing that expensive change (which would also increase weight) just for more bearing life.
Originally Posted by RogerDat
A 5 bolt hub is pretty much required to get 14 or 15 inch tires. If purchasing an axle I think the 5 bolt hub has a larger spindle and takes an 1 1/16 inch bearing which is larger than the 4 bolt spindle 1 inch bearing.
5-bolt hubs are available for the same axle, fitting on the same bearings. The Dexter application catalog lists both 4-bolt (4 on 4.00 bolt circle) and 5-bolt hubs (5 on 4.50 bolt circle). The page of hub parts is headed "440 AND 545 HUB GROUP" (the "440" means 4 bolts on 4" circle
and the "545" means 5 bolts on 4.5" circle
), and the illustration happens to show the 5-bolt hubs.
I wouldn't change axles to get 5-bolt hubs, but maybe if I were buying new brakes
and hubs/drums anyway I would make the change... if those hubs are readily available. Of course, that would require a wheel change, even if staying with the same tire size.
By the way, Dexter currently uses L44649(cone)/L44610(cup) for both inner and outer bearings on the Torflex #9 (up to 2200 pound capacity) axle (and even the size smaller than that); they describe the spindle these go on as 1 1/16" (not 1"). This same size is used as the outer bearing on the 3500 pound axles, but the 3500 pound have a larger inner bearing. I think 1" bearings were previously used by Dexter, but now are found only on some other (typically cheaper) brands.