The Grav Armor rules I posted for FUBAR in the files section are really meant to be for futuristic tanks, where they are quite durable to enemy fire.
The vehicles do have self-protective systems, unlike today's tanks, which defeat many of the inbound rounds, in addition to their armor.
If the same damage roll is made a subsequent time, then you go up to the next higher result. Also, the self-protection systems are degraded by one level, for every hit on the appropriate face of a vehicle (front, left side, right side, rear), due to the use of the self-protection systems on it (limited self-protection ordnance), and also assuming that some of the non-penetrating, or defeated shots scrub some of that off (think shrapnel, degraded beam energy, etc. still doing some minor damage).
For quicker games, you can just ignore the self-protection die rolls, and roll straight against their armor.
Also, the tank armor and weapons stats I provided differ from one another, by tech level, and vehicle size. There are also different armor levels for front, sides, and rear of the vehicles.
Heavier weapons get + modifiers to their die rolls, and light weapons get minuses.
Lower tech levels have less effective armor and weapons ratings, generally.
You compare the FP of the weapon to the armor. If the FP of the weapon is 5 and the armor rating is 3, you'd add 2 to the base damage die roll.
The rules I submitted were suggested for platoon to company sized actions, and I think they work rather well for that. Given only 3 – 15 vehicles, or so per side, you don't want the armor being killed off too quickly, or you'll only have a ten minute game.