Will 255/85 16 tires fit on 16x5.5 split rims?

Yes; things will be a bit tight. But (I'd guess you are putting them on a Land Cruiser) why don't you get some 8.25R16 Michelin XZLs? 1" tall, 1" narrower, far stronger, better tread than any 255/85R16.
Charlie

Yep, they are fitted on a BJ75 troopie.
How are the XZL in mud compared to BFG MT, Cooper STT and other M/T tires?

I second Charlie's motions, for your application the Michelin's would be a better fit to the split rims, and there is simply no comparison in terms on strength and reliability between them and the commercially-available tires.

Good luck!

As above: the 8.25R16 XZL is superior in mud traction a FAR superior in strength (sidewalls and tread) to a BFG 255/85R16 M/T. Where are you? Europe? Namibia? Oh yeah, Sweden.
Charlie

Europe-> Scandinavia-> Sweden...
The XZL 8.25 costs only 200$ in Sweden:) compared to MTR and BFG MT 35s that cost almost 300$ each :(

My God! There's not even any competition then!! I'd pay twice as much for XZLs over BFG trash!
Charlie