increase tire size

When I moved to oversize tires I replaced the wheels with new ones with the correct offset.

In 2013, I replaced two of my E-range tires with two F-range, and in 2013 upgraded the other two (putting on one F-range and one G-range.) The spacing looks to be about an inch and a half. I have had no problem with tires after making the replacements.

Dexter axle mfg recommends a 1" minimum between the tires with the std leaf spring suspension.

Some say 1" is the absolute minimum for clearance. As tires heat up, they expand/air up a little, and yes there is a little movement ahead/back from suspension. If there is wear/slop in suspension, even more so. IMO, 1.5"s of clearance should work, if suspension components are solid.

Jerry

an inch an a half is fine between the tires.... as for the comment about "what if something gets kicked up between them?".... well, what if it happens right now the way they are?... its going to get kicked out of the way because one tire face is going up and the other is going downward, so it cancels each other out.. but gravity is still in your favor so the debris will never get caught between the two opposing directions and go into a perpetual spin between the tires...

yes, the tires can move a little, and the center axle of a tridem much more so than a tandem axle set up. but its going to be a rare instance where in normal operation the axle moves over an inch fore and aft.... and this "rare inch" of movement includes a half inch forward from normal to a half inch rearward from normal.... any axle that can move a full inch rearward/forward from is normal location means the suspension is faulty.

They absolutely can move closer together.
As the equalizer pivots one direction, the effective distance between them changes.
Anyone who uses a something like the BAL tire locking chock knows how much they move inward and outward depending on incline.

I know that the tires can not move any closer together and will normally never touch, but what if something gets kicked up by the front tire? There is no room for the object to pass between. I would worry about a catastrophic failure of possible both tires. I own a machine shop so it will not be difficult to machine a new spring equalizer that is eight inches on centers instead of my current one which is six inches. That, in addition two a mod to the two outside attach points for the springs, will put me back at the current 3.5 inch clearance. While I can do all the design and machining, I can't help wondering what other people are doing, or if it is just my trailer who has this narrow 33.5 inch axle spacing.

An inch is as good as a mile.