First the easy thing. The Fire Truck itself just looks perfect sizewise. If you don't feel comfortable though, I'd recommend you to get wheels, which are a bit bigger and with a structured profile. If you google Firetruck you'll see that the wheels are slightly out of scale.
Second. I see both barrels problematic. Proportionswise the left one is for its heigth to wide and the right one to slim (actually just the body. together with the Wire its pefect, but the rims are just to big itself)
So you have the choice between making the left one higher or reducing the diameter of the wire for the right one and increasing the bodywidth a tiny bit (Or make it a bit shorter). Obviously the first would be easily accomplished with your already used materials and offers compensation for the base and good LOS block while the latter would be more in scale.
Another Idea would be to try to blend the wire of the right barrel "more in" through sheets of cardboard between the wire or some kind of thick coating like gesso or liquid plastic.
The GW barrels are 2,5cm : 1,7cm. Just for reference.
An all alternative method would be to glue 4 pieces of wire in the length of the perimeter of the barrels plus 2mm to cardboard. Then cut some thick aluminium foil to size overlapping on top and bottom off the wires and emboss it with the back of a pencil. After removing the foil from the wire bend the excess ends over and bend together. It looks very homogenous as well and after setting up your template mass production is relatively easy. The big plus is you can have smashed, battered or empty barrels this way.
I know I get excessive sometimes... Sorry guys.