I thought the grapple ones were the easiest.
Just grapple the magnet close, drive slowly, and grab lots and lots before deposting. It should be possible to get 10-20k ore in a single go, which will easily leave you at 3.5-5 gears.
Police don't turn up until you reach 3 gears, and you should get like 30 extra seconds, leaving you with 1-2 mins left.
Just do another slow loop, focussing on big piles and warehouses, and you should be done, probably with time to spare.