Driving XP farming is actually horrid after level 15.

A trip from spawn to a FOB with 2 pallets gives you 1500 driver xp with the big truck.

But the thing is that most of the levels after that point requires you an average of 30k XP to get a level. So around 20-ish trips. PER LEVEL

So if you wish to get the armored humvee with the turret in order to get better driving XP, you need to to do 100 trips more or less. since it's unlocked at level 20.

Let's say it's an average of 6 minutes to do a trip, do it 100 times. you got yourself 10 hours of LITERALLY just driving a car. You better prepare a big watch list on youtube or something on netflix because the farm is really tedious.

that's 10 hours if EVERYTHING goes in your way without any issues, so you can expect maybe 12-15 hours in the end.

I can't imagine grinding for the rest of the vehicles.