So this is really just for fun and represent even bigger explosions that might take place from excessive rolls on the vehicle damage tables. It wouldn't happen too often but it'd be good for a bit of fluff I suppose.
Any result on the damage table that would be greater than a

adds the additional amount to the radius of the explosion.
Example 1) A melta gun penetrates a Land Raider and rolls a

for an explodes result. Since the result would actually be a 7 (from the +1 AP1) you would roll
D6+1 for the radius.
Example 2) A Medusa with Bastion Breachers penetrates an open topped Ork Vehicle. Rolling a 5 on the damage table would become a 7 (+1 for AP1 and +1 for open topped) and so the radius would again be
D6+1.
You could get a maximum of
D6+2 (unless there's something other than AP1 and Open-topped that I'm forgetting) so it wouldn't be catastrophic, but would both be a bit of a moral victory for REALLY popping that vehicle
Then again, 8 inches will catch quite a few more of my Meltavets in the boom...
DoW