I've been looking for wheel hubs for the rear wheels, but never found a good match. Turns out the truck has reduction hubs, with planetary gearing inside, and that's why they're so big.
https://www.scania.com/group/en/wp-c...res-033241.jpg
I've also been looking for a better match with the wheels. I believe Tamiya's rear wheels are made to match the front wheels of the American trucks like the King Hauler, and after some research I believe those are modeled after 24.5 inch wheels with a 285.75 mm bolt circle. That means the scaled down bolt circle is very tiny compared to the European style 22.5 inch wheels with a 335 mm bolt circle that I'm using for front wheels.
The real fire truck uses 20 inch wheels, so the perfect scale version would probably use 1.55 rims, but I'm not going to get custom wheels 3d printed and cut down the tires. What I will try is to do some basic modifications to the Tamiya rims. I've mocked up a couple of different sizes of the hub to find a good looking compromise between what's actual scale and what looks scale.
(And for reference from the first page: )
edit:
I think this will work
