Hands down absolutely positively the best vehicle design system - particularly for AFVs - is Dream Pod 9's Silhouette System.
First you start off with a desired size, and speed, and desired range.
Speed is very easy to do - Silhouette's hexes are 50m. Size is usually figured last; adding weapons and various capabilities increases the size and if you want a smaller frame you pay for it on the back end when you're figuring everything up.
Range is a perk that's added up later during configuration.
The coolest part is armor because its based on real-world systems so you can model anything from an Imperial German A7v tank to an M1A2 TUSK (and beyond). What you do is you take the square root of the vehicle's frontal armor thickness (or equivalent) millimeters. That's your base armor rating.
So in the case of the Abrams if we assume about 650mm of RHA equivalence then the square is 25.
Here's the significance of that: Every weapon in the game has a base Damage Multiplier. Every point of success over the target number needed is multiplied by that number. The total is then compared to the base armor of the vehicle. If the total damage is equal to or less than the base armor of the vehicle, no damage is done. If the total is greater than 1x the base armor but less than 2x, a LIGHT hit is done, and the vehicle's base armor is reduced by 1 and the Light Damage table is consulted. If the total damage done is 2x the base armor rating, but less than 3x, the Heavy Damage table is consulted (where all kinds of horrible things can happen), and the vehicle's base armor is reduced by 2. If the damage result is more than 3x the base armor rating then it's Overkilled and the vehicle is destroyed.
Here's an example...let's take a T72, which per the rulebook (they use the '72 as an example in the book even though the game is sci-fi). They say the BAR is 15...so it's armor ratings are 15/30/45
A TOW is fired from an ambush position at the T72. The gunner firing the TOW has a good gunnery rating, so he rolls 2 dice. The '72 isn't sporting reactive armor for our example. The Difficulty needed to hit is, after all modifiers (range, movement) a 4. The TOW gunner rolls 2 dice, and because he's average in his stats, adds no further modifiers. He rolls a five and a six. As the six is the higher die, he uses it as his "success" die. The 6 is two points higher (a margin of success) than the target of 4, so it's a hit!
The TOW is a Heavy Anti-Tank Missile, which has a Damage Multiplier of x20. The margin of success success of +2 is multiplied by this for a total of 40. Ouch! The T72 takes a terrific wallop.
The 40 points is greater than 2x the base armor rating of the target, but (only just!) less than 3x. We roll 1d6 and get 4: Structural Damage, check the Heavy chart, and see that the damage done is -2 to Movement (all movement - acceleration, turning, etc.)
So our now very hurt T72 has a Base Armor rating of 13 and is going to be pretty slow to move from here on out...
Sorry, I hope I didn't oversell that, and the system is a LOT more elegant than I'm making it out to be.
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