That's not really a good idea.. placing most of the vehicles in just one active police department wouldn't be that good. Being balanced, as it currently is, would be great.
LCPD cannot have more vehicles added, and by your suggestion, more cruisers should be moved to DPD which means that FIAPD at first will have no more LCPD Buffalos, and at second the lack of cruisers at the rest of PDs.
I don't really know how would this help, as I don't see anything that would help besides filling up the PD and causing troubles when escorting suspects.. Unsupported.