Medium Duty Ambulance Conversion, another post on the topic of tires/wheels and weight limits.

[AuroraBorealis]

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[AuroraBorealis]

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[billiebob]

this truck will be used all 4 seasons, and it will definitely see snow including the potential for a sudden 12 inches overnight

So stay close to stock, keep the rear duals, not super single conversion, and buy tire chains.
Whatever changes you make be sure you can easily put the chains on.

A lot of the choices you list involve massive unsprung weight increases which equal rougher riding.
For the travel you do, in Ontario, 4WD is not a neccessity. Chained up duals will be more effective.

The 19.5 rim is for low profile tires. Jumping up to 22.5 or 24...... will definitely need a regear.... will they even fit? They will be wider, they will be WAYY taller.... And they will be much heavier. 22.5 and 24 tires are built for TRUCKS, not ambulances.
Stepping down to 17 or 18 will let you go to a taller profile and keep the same basic tire height IF the brakes will allow smaller rims.
If the rims fit over the brakes, 17" will give you a massive tire selection and likely more economical if they have the correct load rating.

You need to talk to your local tire shop and find the tire you want first.

 

[AuroraBorealis]

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[AuroraBorealis]

Bump, not sure if bumping works or not but it cant hurt.

 

[Ozrockrat]

• Buying a 8 lug rim in 18"/20"/22.5"/24.5" e.t.c
  • Do these rims not exist or was it just like finding a needle in a haystack? "Super Singles"These rims do exist but are very rare. Check places that have logging trucks or the like of Michigan, also they did seem to use a lot of them in Florida on construction sites for the likes of cement trucks. The problem is that most manufacturers stepped away from 8 lug axles in the late 90's so supply is just not there. You can buy them new but expect to pay $350 and up each. But these give you the best option for tires availability. And it will be new tires not the dried up ones like usually sold surplus. Another benefit is that you would be around 41-42" for 22.5 super single tires (wide base as they call them here in the USA) which would suit your current gearing better. "Dualies" 8 lug 22.5 dualie style rims are more readily available and cost effective. You still have the same issues with sidewall hardness etc that you do with the current 19.5 setup. Tires are readily available and more reasonabily priced. This would probably be my recomendation for your use case. (extra clearance without engineering headaches)
• Altering my tires to MRAP, or some military surplus tires.
  • Military surplus tires in 43" are like rocking horse poop. Impossible to find and expensive when you do. Expect them to be 10+ years old ( as far as I know old tires are a problem in Ontario). I have had mixed success with obtaining/using them. Continental MPT's are available new but expensive (like any large tire). The weight rating for your truck would allow either 335/80R20 or 365/80R20 (41" or 43"). Either of these tire options requires an MPT profile rim. Very hard to find surplus but some companies are adapting the axletech size rims to suit ($650-$700 ea). To convert the rims yourself involves machining the bead area and manufacturing an adapter plate (see Driven to Adventures on Youtube for an glimpse of those rims/tires)
• Replacing all Axles with Axletech's and converting the vehicle to 4x4 at the same time, along with military tires.
  • This is not rocket science but also not something to be undertaken lightly. Axles are getting more expensive and hard to find. Even with buying my axles etc back in the cheaper days I think I will be around $15K invested in this modification. Plus a lot of blood, sweat and swearing. It was OK doing it on my driveway with the tolls etc I have access to (little electric forklift and truck sized jacks and stands plus big air tools) but I would not consider it in the bush. There are other front axles available that can be matched to your existing rear axle but it will still be expensive and time consuming. Western Canadian Rockwell (Neil) would be the source for axles/parts in Canada.
• Getting a 10 lug adapter so I can buy 10 lug military wheels today, and then doing the axle/4x4 conversion in the future.
  • One of my mantra sayings is "Just because you can doesn't mean you should" . I will just say I would never recommend using adapters where there is any other option. (spacers between dual wheels is sort of acceptable).

 

[Ozrockrat]

Allison transmission - Grab the serial number off your transmission and go to My Allison to work out what you have and what options it was built with. I would be surprised for it to be a 1000.

 

[AuroraBorealis]

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