mod version used; 0.2.3, factorio version used; 0.15.18
testing; i did build a 5*2 block of (regular) walls and tried to destroy them by ramming the flame tumbler into them, without repairing the vehicle.
took me 1 minute to do and left the flame tumbler with 399/600 health. you can also ram down a complete player build base with this vehicle easily without even repairing it. (assuming it doesnt have walls or defense structures active)
suggestion; while i appreciate the idea of this vehicle, it's kinda overpowered in terms of taking down forests/buildings while obtaining extremely high speeds, especially in terms of multiplayer.
i suggest to reduce impact-resistance (percent value) to 50, leave the rest as it is. this will do ~6 damage when hitting a tree (1% of current unit HP) and ~40(~6,66%) of current unit HP) when hitting a rock.
(this'll make it a bit less tanky than a regular car and adds some realism due to the fact the tumbler has outstanding maximum speed and acceleration. higher speed => harder impact)