OK, not a car, but still pretty silly. A RR on a light cycle!
Wasp 150 by Swordtart
Light Cycle; Cycle chassis; Heavy suspension; 10 cid Engine; 3 gal. Hvy-Duty tank; 2 Hvy Duty OR tires. Driver. Recoilless Rifle w/10xHEAT. Tripod mount for RR. 8 pts. Plastic (F: 6 R: 0 L: 0 B: 2 T: 0 U: 0).
Cost: $3,630, Wgt: 787, HC: 2 (3 OR), Top Speed: 65, Accel: 5.
Note that the RR is a tripod weapon mounted alongside the driver. Tripod weapons take up an extra space in a crew position, so I believe that this is legal.
It's even a real thing (or as close as I can get in CW rules).
https://en.wikipedia.org/wiki/Vespa_150_TAP
With the negative modifiers for drivers firing hand weapons from a moving vehicle, this looks pretty silly, but park up and with the braced and stationary bonuses it becomes a credible threat to other similarly priced vehicles.
It doesn't have nearly enough armour (an eggshell armed with a hammer) so you'll want to keep terrain between you and any opponent, but it will be harder to hit than many targets. OR tires mean you can find a place where you are hard to get at and extract (where your low top speed won't hinder you). You also have the option of moving the RR to an entrenched position if you want to use it more conventionally (but that might hinder extraction and in CW it makes little difference).
In a small arena? - nah, it's toast :)
Add a laser scope and it might even outclass many other div 5 designs. As an ambush vehicle it is cheap and it is also suitably Heath-Robinson for a dreg biker.
In reality this was supported by a companion scooter which carried extra ammunition. On the second bike, swap out the Tripod RR for a passenger that has maxed out on Gunner skill and you have a three man fire team with a decent shoot and scoot capability. There is plenty enough weight left over for ammunition, extra hand weapons and BA for all crew so depending on your budget you could have a handy little element ideal for taking out couriers, cutting out escorts or harassing even quite well armed convoys.