Ramming Enemies (page 95, Mutant Year Zero book) says the attack would occur at Arm's Length and be a Fight attack, but instead using your Agility stat and Move skill. The weapon damage is 1 (though this can be modified by a Gearhead, like putting spikes on the car). This damage applies to occupants of the rammed car, and the damage to the rammed vehicle would get multiplied by the ramming car's resilience. NOTE: the attacking vehicle has to have an equal or higher resilience to perform the attack.
An Explosive, at Arm's Length, is going to do 2 damage base (unless there's shrapnel, in which case it's 3), and at a bare minimum have 6 attack dice. A Gearhead can make a base 6 explosive with a success, use 1 stunt for shrapnel, and add 3 attack dice for each stunt.
Let's assume our players (driver and bomb maker) are lucky and get 4 successes on 7 dice (very lucky indeed).
The automobile attack at Arm's Length will do 4 damage to everyone in the car (1 base damage, plus 3 Fight stunts used for damage), and 8 damage to the rammed car (both minus armor).
The bomb at Arm's Length will have 15 attack dice (6 base plus 3 stunts of 3 dice each), which if we get 4 successes for every 7 dice, would result in about 9 damage (2 base, plus 7 more successes) to everyone in the car, as well as the car (minus armor of course).
There's some assumptions here, and of course you can push a Fight roll (for the automobile), and can't push an explosive roll (see page 94). But all in all, they seem fairly comparable in damage.