an interesting post for md as it is something I have been considering for my own truck. I have a 4x2 merc 1823 and it is currently fitted with 315/80R22.5 tyres on 9x22.5 steel rims.
The rear tyres are Bandag retreads on Hankook carcasses. The tread is a good compromise between traction and road manners (designed for landfill work) and the retreads have proven to be great, but I do not want to leave my trust in these tyres when I set off for China in a year or so.
I am not interested in changing to 20inch rims and I'm not sure if it is possible with the large disc brakes I have and so my options are similar to yours.
I like the 315/80 size as, apart from the 295/80, it is probably the most widely used truck tyre size. It makes getting replacements extremely easy, but the downside is that the choice of off-road biased treads is limited (although the XZY2 does come in this size) and that I am stuck with the twin rear wheels.
The alternative tyre size is the 13R22.5 which you had. I have been offered the Michelin XZL at a good price in this size which is what tempted me to it, but the size is far less common than the 315/80 (although it is very similar in size), and apart from the better choice of off-road biased treads there are no other advantages.
My truck, being 4x2, is obviously more limited in how far off the road I can travel, but as yet I haven't had any problems at all with the twin rear wheels or the 315/80 tyres generally. The front tends to sink in in soft ground but as the front axle is not powered it makes less issues for me I guess. Given the amount of money you have just spent you thought they were a problem on your vehicle. What problems were you having with the twin wheels and the 13r22.5 generally?
The concerns I have about changing to wider tyres on my truck are the derating of the rear axle, the stress on the front axle, and the difficukty in sourcing replacements.
The rear axle on my truck is 13t, and whilst I will never need this much capacity (current wieght is less than 6t) it seems silly to make the tyres and wheels the weak link.
Another concern is with regard to the oversize front wheels. The largest tyres size Mercedes offer on my 8t front axle is a 385/65 and the size I would need to make the back axle usable would be far larger than this at the front. Haulage firms in the UK stay away from super singles at the from due to increased stearing wear and poorer ride comfort, but this may just be anecdotal. Has anyone advised you about putting such large wheels and tyres on your (7t?) front axle?
I was also slightly concerned that these extremely wide 22.5 tyre are specialist and will not be stocked anywhere. I carry two spares and my tyres typically outlast my truck ownership anyway but it is still worth considering.
I am currently leaning towards sticking with the 315/80 and getting XZY2 at the back and XDY3 at the front, but I am loooking for experiences that may pursaude me otherwise.
Thanks for a great post and any help you can give me.