Does d20 Modern have the same DM advice regarding terrain features as DnD? I mean the "if terrain provides a significant advantage to the monsters, increase the encounter level (and thus the XP reward) by at least 1 level, possibly more" suggestion. You could use that rule as a baseline for adjusting encounter levels.
A sniper hidden somewhere in a 10 story building is much more dangerous than a sniper standing in the middle of the road. A pilot is much more dangerous inside his combat helicopter than out of it - and the same applies to the tank crew.
Perhaps you could apply a flat +1 to the encounter level when a pilot is inside the appropriate vehicle, and an additional +1 to +3 depending on the vehicle's weaponry/defenses (e.g., +1 if the PCs have anti-tank weapons and the tank lacks reactive armor, or +3 if the PCs don't have anti-tank weapons and the tank has additional anti-infantry weapons).
So a tank crew inside a tank not specifically protected from the PCs would have an encounter level of CR+2, while a tank crew inside a tank specifically protected from the PCs and armed against them would be CR+4. Assuming that
CR == average party level this would make for a very dangerous fight and the best idea would probably be to try to outmaneuver the tank or flee from it (I sure wouldn't want to take on a tank head on!).